chore(theming): remove Sass default flags (#29401)

Issue number: N/A

---------

## What is the current behavior?
Currently, every Sass variable in Ionic has the `!default` flag added to
the end.

From the [Sass variables
documentation](https://sass-lang.com/documentation/variables/):

> Normally when you assign a value to a variable, if that variable
already had a value, its old value is overwritten. But if you’re writing
a Sass library, you might want to allow your users to configure your
library’s variables before you use them to generate CSS.
> 
> To make this possible, Sass provides the `!default` flag. This assigns
a value to a variable only if that variable isn’t defined or its value
is [null](https://sass-lang.com/documentation/values/null). Otherwise,
the existing value will be used.

In past versions of Ionic Framework, developers wrote Sass variables to
rebuild Ionic Framework using their own values. In the latest versions
of Ionic Framework, this is not possible.

## What is the new behavior?
Removes the `!default` flag from all Sass variables.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

-------

Co-authored-by: brandyscarney <brandyscarney@users.noreply.github.com>

Author: liamdebeasi

core/src/components/accordion/accordion.md.vars.scss

@@ -4,10 +4,10 @@
 // --------------------------------------------------
 
 /// @prop - Border radius applied to the accordion
-$accordion-md-border-radius:         6px !default;
+$accordion-md-border-radius:         6px;
 
 /// @prop - Box shadow of the accordion
-$accordion-md-box-shadow:            0px 3px 1px -2px rgba(0, 0, 0, 0.2), 0px 2px 2px 0px rgba(0, 0, 0, 0.14), 0px 1px 5px 0px rgba(0, 0, 0, 0.12) !default;
+$accordion-md-box-shadow:            0px 3px 1px -2px rgba(0, 0, 0, 0.2), 0px 2px 2px 0px rgba(0, 0, 0, 0.14), 0px 1px 5px 0px rgba(0, 0, 0, 0.12);
 
 /// @prop - Margin of the expanded accordion
-$accordion-md-expanded-margin:       16px !default;
+$accordion-md-expanded-margin:       16px;

core/src/components/accordion/accordion.vars.scss

@@ -4,16 +4,16 @@
 // --------------------------------------------------
 
 /// @prop - Background color of the accordion
-$accordion-background-color:      var(--ion-background-color, #ffffff) !default;
+$accordion-background-color:      var(--ion-background-color, #ffffff);
 
 /// @prop - Duration of the accordion transition
-$accordion-transition-duration:   300ms !default;
+$accordion-transition-duration:   300ms;
 
 /// @prop - Timing function of the accordion transition
-$accordion-transition-easing:     cubic-bezier(0.25, 0.8, 0.5, 1) !default;
+$accordion-transition-easing:     cubic-bezier(0.25, 0.8, 0.5, 1);
 
 /// @prop - Opacity of the disabled accordion
-$accordion-disabled-opacity:      0.4 !default;
+$accordion-disabled-opacity:      0.4;
 
 /// @prop - Margin of the inset accordion
-$accordion-inset-margin:          16px !default;
+$accordion-inset-margin:          16px;