[ubsan-minimal] update comments in CMakeLists.txt

Summary:
Prepend minimal when UBsan is mentioned and delete a dead comment

Patch by Igor Sugak

Reviewers: eugenis, pcc

Reviewed By: eugenis

Subscribers: mgorny, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D53826

llvm-svn: 345557