imagesnap: remove explicit SDK setting from xcodebuild
imagesnap: remove explicit SDK setting from xcodebuild

This "-sdk" flag no longer seems to work; this meant that this
formula no longer builds from source and didn't get a Big Sur
rebottle.

The flag was originally added to this formula in 2011 to work
around some issue with Xcode 4 (3c8f29684e8b0842792d8312090f8bdcf5855a7c)
It seems to be the only formula that does this so it seem safe
to remove.

Also mark it as explicit public domain per:
  https://github.com/rharder/imagesnap/blob/master/README.md#a-note-about-public-domain

Closes #71132.

Signed-off-by: Alexander Bayandin <673dbf9b1367181cd47bae83bf10b2ffe51be6ac@gmail.com>
Signed-off-by: BrewTestBot <8a898ee6867e4f2028e63d2a6319b2224641c06c@users.noreply.github.com>

stgraber/homebrew-core,zyedidia/homebrew-core,adam-moss/homebrew-core,Moisan/homebrew-core,filcab/homebrew-core,JCount/homebrew-core,cblecker/homebrew-core,sjackman/homebrew-core,JCount/homebrew-core,filcab/homebrew-core,Linuxbrew/homebrew-core,lemzwerg/homebrew-core,sjackman/homebrew-core,Homebrew/homebrew-core,makigumo/homebrew-core,mbcoguno/homebrew-core,uyjulian/homebrew-core,adam-moss/homebrew-core,mbcoguno/homebrew-core,zyedidia/homebrew-core,Homebrew/homebrew-core,mbcoguno/homebrew-core,makigumo/homebrew-core,lemzwerg/homebrew-core,uyjulian/homebrew-core,Linuxbrew/homebrew-core,mfikes/homebrew-core,Moisan/homebrew-core,spaam/homebrew-core,nandub/homebrew-core,chrisfinazzo/homebrew-core,mfikes/homebrew-core,nbari/homebrew-core,spaam/homebrew-core,stgraber/homebrew-core,dsXLII/homebrew-core,nbari/homebrew-core,chrisfinazzo/homebrew-core,nandub/homebrew-core,dsXLII/homebrew-core,cblecker/homebrew-core