Placeholder: remove unused and vendor specific CSS

[dmitryrogozhny]

Q	A
Bug fix?	[x]
New feature?	[ ]
New sample?	[ ]
Related issues?	fixes #X, partially #Y, mentioned in #Z

What's in this Pull Request?

A small enhancement to Placeholder control styles to make the code a bit cleaner:

• Removed .placeholderOverlay and .placeholderSpinnerContainer styles which are not being referenced anywhere in the code.
• Removed vendor specific styles, such as display: -webkit-box; as they got automatically created by SPFx build process.

As for vendor prefixed styles properties, excerpt from the Recommendations for working with CSS in SharePoint Framework solutions article:

In SharePoint Framework, no vendor prefixed style properties are required in the Sass or CSS files of a project. If some of the SharePoint Framework-supported browsers require prefixes, they were added after the Sass to CSS compilation automatically.

[codecov-io]

Codecov Report

Merging #426 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #426   +/-   ##
=======================================
  Coverage   73.31%   73.31%           
=======================================
  Files          18       18           
  Lines         802      802           
  Branches      164      154   -10     
=======================================
  Hits          588      588           
  Misses        164      164           
  Partials       50       50

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[AJIXuMuK]

Thank you @dmitryrogozhny for the update!