Remove gawk from the build and let one-true-awk be our default awk from now

Remove gawk from the build and let one-true-awk be our default awk from now
on. [1]

Let the bootstraping build one-true-awk and not gawk, and add maketab.nx to
CLEANFILES.

Submitted by: ibotty <bsd@ibotty.net> [1]