pgcrypto: Remove unused binary from clean target
Generation of the gen-rtab binary was removed in
db7d1a7b0 but it
was accidentally left in the cleaning target. Remove since it is
no longer built.
Author: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/
c1d63754-cb85-2d8a-8409-
bde2c4d2d04b@gmail.com