[ARM] Remove ThumbTargetMachines. (NFC)

Summary:
Thumb code generation is controlled by ARMSubtarget and the concrete
ThumbLETargetMachine and ThumbBETargetMachine are not needed.

Eric Christopher suggested removing the unneeded target machines in
https://reviews.llvm.org/D33287.

I think it still makes sense to keep separate TargetMachines for big and
little endian as we probably do not want to have different endianess for
difference functions in a single compilation unit. The MIPS backend has
two separate TargetMachines for big and little endian as well. 

Reviewers: echristo, rengolin, kristof.beyls, t.p.northover

Reviewed By: echristo

Subscribers: aemerson, javed.absar, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D33318

llvm-svn: 303733