Fsdk/Misc: obsolete FsxOnlyArgs & rm more !LEGACY #1605

Job	Run time
linux-newLTS-github--dotnet-and-xbuild	7m 39s
linux-newLTS-github--dotnet-and-msbuild	7m 41s
linux-newLTS-vanilla--newmono	7m 24s
linux-newLTS-vanilla--dotnet6	7m 49s
linux-newLTS-vanilla--stockmono	5m 48s
linux-oldLTS-github--dotnet-and-xbuild	7m 34s
linux-oldLTS-github--dotnet-and-msbuild	8m 2s
linux-oldLTS-vanilla--newmono	6m 36s
linux-oldLTS-vanilla--stockmono	5m 15s
macOS--dotnet6-and-mono	11m 39s
macOS--mono	8m 11s
windows--dotnet6	11m 49s
windows--legacyFramework	14m 59s
sanity-check	0s
publish	0s
	1h 50m 26s

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