Resolves #6974 v-btn font-size overwrite by typography css

[cloudsoh]

Description

Remove the v-btn styling from _typography.styl.
Remove $headings.button from _variables.styl.

Motivation and Context

Fixes issue #6974 .

How Has This Been Tested?

none

Markup:

Template

<v-btn large>Button</v-btn>

_typography.styl

.v-btn
  font-size: $headings.button.size
  font-weight: $headings.button.weight

vuetify.css

.caption {
  font-size: 12px !important;
  font-weight: 400;
}
.v-btn {
  font-size: 14px;
  font-weight: 500;
}

vuetify.min.css

.caption{font-weight:400}.caption{font-size:12px!important} // There's no v-btn

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and breaking changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)

[KaelWD]

The variable was added in e481c08 but never used, and the rule was added in #2397 despite $button-font-size and $button-font-weight already existing.