Remove entab and associated detritus.

We don't need this anymore, because pg_bsd_indent has been taught to
follow the same tab-vs-space rules that entab used to enforce.

Discussion: https://postgr.es/m/E1dAmxK-0006EE-1r@gemulon.postgresql.org
Discussion: https://postgr.es/m/30527.1495162840@sss.pgh.pa.us

Author: tglsfdc

src/tools/entab/.gitignore

@@ -14,9 +14,7 @@ PREREQUISITES:
    git clone https://git.postgresql.org/git/pg_bsd_indent.git
    then follow the directions in README.pg_bsd_indent therein.
 
-2) Install entab (src/tools/entab/).
-
-3) Install perltidy.  Please be sure it is v20090616 (older and newer
+2) Install perltidy.  Please be sure it is v20090616 (older and newer
    versions make different formatting choices, and we want consistency).  See
    https://sourceforge.net/projects/perltidy/files/perltidy/perltidy-20090616/