[ARM] Cleanup so_imm* tblgen defintions

No functional changes. Support for ARM's modified immediate syntax was added
in r223113 and r223115 (review: D6408). That patch introduced the mod_imm*
tblegen definitions which renders the existing so_imm* definitions redundant.
This patch gets rid of them completely.

Reviewed as: D6722

llvm-svn: 225266