Remove more obsolete IE9 filter CSS syntax
Summary:
Same game as rPe38f5457 but hadn't realized before:
IE5.5 to IE8 released 03/2009 supported the `filter: alpha()` syntax.
(IE8 and IE9 supported `-ms-filter` which was deprecated in IE9 and is unsupported since IE10 released 09/2012.)
See
* https://learn.microsoft.com/en-us/previous-versions/windows/internet-explorer/ie-developer/platform-apis/ms532847(v=vs.85)
* https://learn.microsoft.com/en-us/previous-versions/windows/internet-explorer/ie-developer/platform-apis/ms532967(v=vs.85)
* https://learn.microsoft.com/en-us/previous-versions/ms530752(v=vs.85)
* https://stackoverflow.com/questions/40389599/firefox-declaration-dropped-errors
`filter` is a valid standard CSS3 property unrelated to IE's obsolete `filter: alpha()` syntax.
Standard `filter` does not support `alpha()` per https://developer.mozilla.org/en-US/docs/Web/CSS/filter.
Thus modern browsers throw errors in the console to be avoided by removing this old syntax.
Test Plan: Read the links above, realize it's been 16 years, and agree with me.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D25930