[lldb] [gdb-remote] Remove HardcodeARMRegisters() hack

HardcodeARMRegisters() is a hack that was supposed to be used "until
we can get an updated debugserver down on the devices".  Since it was
introduced back in 2012, there is a good chance that the debugserver
has been updated at least once since then.  Removing this code makes
transition to the new DynamicRegisterInfo API easier.

Differential Revision: https://reviews.llvm.org/D111491