[ARM] Remove a redundant function fixupBTI

Since the redundant BTI instructions emitted by jump tables are now
removed in the ARMBranchTargets pass, the fixupBTI function is not needed
in the ARMConstantIslandPass. Some related tests are removed as well.

The relevant patch that removes the redundant BTI instructions:
https://reviews.llvm.org/D144470

Differential Revision: https://reviews.llvm.org/D145048