[MC] Delete MCCodePadder

D34393 added MCCodePadder as an infrastructure for padding code with
NOP instructions. It lacked tests and was not being worked on since
then.

Intel has now worked on an assembler patch to mitigate performance loss
after applying microcode update for the Jump Conditional Code Erratum.

https://www.intel.com/content/www/us/en/support/articles/000055650/processors.html

This new patch shares similarity with MCCodePadder, but has a concrete
use case in mind and is being actively developed. The infrastructure it
introduces can potentially be used for general performance improvement
via alignment. Delete the unused MCCodePadder so that people can develop
the new feature from a clean state.

Reviewed By: jyknight, skan

Differential Revision: https://reviews.llvm.org/D71106