Simplify null-element handling in extension_config_remove().

Simplify null-element handling in extension_config_remove().

There's no point in asking deconstruct_array() for a null-flags
array when we already checked the array has no nulls, and aren't
going to examine the output anyhow.  Not asking for this output
should make the code marginally faster, and it's also more
robust since if there somehow were nulls, deconstruct_array()
would throw an error.

Daniel Gustafsson

Discussion: https://postgr.es/m/289FFB8B-7AAB-48B5-A497-6E0D41D7BA47@yesql.se