a2e35b53 added unused variable to ConversionCreate()

I'm seeing this on a the master branch tip when building at -O2:

pg_conversion.c: In function ‘ConversionCreate’:
pg_conversion.c:53:6: error: variable ‘oid’ set but not used
[-Werror=unused-but-set-variable] Oid   oid;     ^
cc1: all warnings being treated as errors

I think that commit a2e35b53 did this.

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Peter Geoghegan