MIR Printer: Remove an outdated TODO comment and assertion. NFC.

This commit removes an outdated TODO comment and a corresponding assertion
which asserts that the mir printer can't the print machine basic blocks that
aren't sequentially numbered.

This comment and assertion were correct when I was working on the patch which
serialized the machine basic blocks, but then I decided to add an 'ID'
attribute to the machine basic block's YAML mapping based on the patch review.
This comment and assertion then became invalid as with the 'ID' attribute we
can serialize the non sequential machine basic blocks and their references
without any problems.

llvm-svn: 243447