[llvm-readobj/llvm-readelf] Delete --arm-attributes (alias for --arch-specific)

D68110 added --arch-specific (supported by GNU readelf) and made
--arm-attributes an alias for it. The tests were later migrated to use
--arch-specific.

Note, llvm-readelf --arch-specific currently just uses llvm-readobj
style output for ARM attributes. The readelf-style output is not
implemented.

Reviewed By: compnerd, kongyi, rupprecht

Differential Revision: https://reviews.llvm.org/D68196

llvm-svn: 373291