Remove some utility mixins

What does this MR do?

This removes a few utility mixins so that Tailwind can take precedence.

• appearance.scss
  • The gl-appearance-none has been removed.
  • Internal usages of its have been migrated to Tailwind's @apply directive.
  • Consumers will need to enable the appearance Tailwind core plugin.
  • No renames necessary.
• content.scss
  • This only defined the gl-content-empty utility mixin which is only used internally.
  • Renamed gl-content-empty to content-empty and moved it to the internal mixins file.
  • Deleted content.scss so that we don't generate utility classes for it anymore.
  • Consumers can enable the content Tailwind core plugin.
• masks.scss
  • This only defined the gl-mask-chevron-down utility mixin which is only used internally.
  • Renamed mask-chevron-down to content-empty and moved it to the internal mixins file.
  • Deleted masks.scss so that we don't generate utility classes for it anymore.
• mix-blend-mode.scss
  • This only defined the gl-mix-blend-mode-luminosity utility which we don't use anywhere.
  • Deleted mix-blend-mode.scss.
  • Consumers can enable the mixBlendMode Tailwind core plugin.
• table.scss
  • This only defined the gl-table-layout-fixed utility which we use in GitLab.
  • Deleted table.scss.
  • Consumers should enable the tableLayout Tailwind core plugin.
  • Rename gl-table-layout-fixed to gl-table-fixed.
• user-select.scss
  • Defined gl-user-select-none and gl-user-select-text utilities.
  • Internal usages have been migrated to Tailwind's @apply directive.
  • Deleted user-select.scss.
  • Consumers should enable the userSelect Tailwind core plugin.
  • Rename gl-user-select-none to gl-select-none.
  • Rename gl-user-select-text to gl-select-text.

Re #2391 (closed), #2398 (closed), #2405 (closed), #2410 (closed), #2414 (closed)

Screenshots or screen recordings

Integration merge requests

• GitLab: gitlab!145433 (closed)
• [-] CustomersDot: mr_url
• [-] Status Page: mr_url

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