Simplify the part of args() that is handling .Bl -column phrases:

Simplify the part of args() that is handling .Bl -column phrases:
Delete manual "Ta" handling because macro handling should
not be done in an argument parser but should be left to the
macro parsers, which exist anyway and work well.
No functional change, minus 40 lines of code.

Confusing and redundant code found while investigating
an old bug report from tim@.