[cmake] Remove confusing condition argument from else() NFC

Summary:
This makes it look like an elseif and also the variable referenced
in the condition was removed from this function in r366622.

Reviewers: dsanders, beanz, smeenai, compnerd, phosek

Reviewed By: beanz

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70159