[clang] NFC: Dead code removal in SemaDecl.cpp, CheckMultiVersionFunction().

This change removes dead code in the definition of CheckMultiVersionFunction()
in clang/lib/Sema/SemaDecl.cpp. The removed code was made dead by commit
fc53eb69: "Reapply 'Implement target_clones multiversioning'".
See the added code just above the code being deleted; it contains the same
return statement with the previous condition now distributed across an if
statement and a switch statement.

Reviewed By: erichkeane, aaron.ballman

Differential Revision: https://reviews.llvm.org/D121955