[clang] NFC: Redundant code removal in SemaDecl.cpp, CheckTargetCausesMultiVersioning().

This change removes redundant code in the definition of
CheckTargetCausesMultiVersioning() in SemaDecl.cpp. The removed code checked
for multiversion function support. The code immediately following the removed
code is a call to CheckMultiVersionAdditionalRules(); that function performs
the same check on entry. In both cases, the consequences of missing multiversion
function support results in the same diagnostic message being issued and the
applicable function declaration being marked as invalid.

Reviewed By: erichkeane, aaron.ballman

Differential Revision: https://reviews.llvm.org/D121957