pgsql: Delete the `remove-old-headers' target, as it is no longer

Log Message:
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Delete the `remove-old-headers' target, as it is no longer useful. Patch
from Alvaro Herrera.

Modified Files:
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pgsql/src/include:
Makefile (r1.16 -> r1.17)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/Makefile.diff?r1=1.16&r2=1.17)