pgsql: Remove useless const qualifier

Remove useless const qualifier

Claiming that the typevar argument to DefineCompositeType() is const
was a plain lie. A similar case in DefineVirtualRelation() was
already changed in passing in commit 1575fbcb. Also clean up the now
unnecessary casts that used to cast away the const.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/66f0cf7da8eeaeca4b9894bfafd61789b514af4a

Modified Files
--------------
src/backend/commands/typecmds.c | 4 ++--
src/backend/commands/view.c | 2 +-
src/include/commands/typecmds.h | 2 +-
3 files changed, 4 insertions(+), 4 deletions(-)