Revert "Fix memory leaks in custom executor (#251)"

[Y--]

This reverts commit 59ada4b.

Seems that the previous PR is causing issues like:

2024-10-07 15:55:16.797 CEST client backend[159884] pg_regress/execution_error STATEMENT:  select a::INTEGER from int_as_varchar;
TRAP: failed Assert("rel->rd_refcnt > 0"), File: "relcache.c", Line: 2176, PID: 159884
postgres: jelte regression [local] idle(ExceptionalCondition+0x6e)[0x5ff6eb3300a4]
postgres: jelte regression [local] idle(RelationDecrementReferenceCount+0x38)[0x5ff6eb323998]
postgres: jelte regression [local] idle(RelationClose+0x15)[0x5ff6eb3239c5]
/home/jelte/.pgenv/pgsql-17beta9/lib/pg_duckdb.so(+0x2b836)[0x7c5b33ae6836]
/home/jelte/.pgenv/pgsql-17beta9/lib/pg_duckdb.so(+0x2bc7e)[0x7c5b33ae6c7e]
/home/jelte/.pgenv/pgsql-17beta9/lib/pg_duckdb.so(+0x2bdbd)[0x7c5b33ae6dbd]
/home/jelte/.pgenv/pgsql-17beta9/lib/pg_duckdb.so(+0x2e015)[0x7c5b33ae9015]
/home/jelte/.pgenv/pgsql-17beta9/lib/libduckdb.so(_ZN6duckdb16AttachedDatabaseD1Ev+0x2d)[0x7c5b2f707edd]
/home/jelte/.pgenv/pgsql-17beta9/lib/libduckdb.so(_ZN6duckdb16AttachedDatabaseD0Ev+0xd)[0x7c5b2f707f1d]
/home/jelte/.pgenv/pgsql-17beta9/lib/libduckdb.so(+0x1409628)[0x7c5b2f009628]
/home/jelte/.pgenv/pgsql-17beta9/lib/libduckdb.so(_ZN6duckdb10CatalogSetD1Ev+0x2e)[0x7c5b2f00c66e]
/home/jelte/.pgenv/pgsql-17beta9/lib/libduckdb.so(_ZN6duckdb15DatabaseManager14ResetDatabasesERNS_10unique_ptrINS_13TaskSchedulerESt14default_deleteIS2_ELb1EEE+0xfd)[0x7c5b2f70830d]
/home/jelte/.pgenv/pgsql-17beta9/lib/libduckdb.so(_ZN6duckdb16DatabaseInstanceD1Ev+0x25)[0x7c5b2f70b5c5]
/home/jelte/.pgenv/pgsql-17beta9/lib/libduckdb.so(_ZNSt16_Sp_counted_baseILN9__gnu_cxx12_Lock_policyE2EE24_M_release_last_use_coldEv+0xe)[0x7c5b2e5d9a6e]
/home/jelte/.pgenv/pgsql-17beta9/lib/pg_duckdb.so(+0x1a6f5)[0x7c5b33ad56f5]
/lib/x86_64-linux-gnu/libc.so.6(+0x47a66)[0x7c5b32e47a66]
/lib/x86_64-linux-gnu/libc.so.6(+0x47bae)[0x7c5b32e47bae]
postgres: jelte regression [local] idle(proc_exit+0x3d)[0x5ff6eb1b013a]
postgres: jelte regression [local] idle(PostgresMain+0x37e)[0x5ff6eb1e0ffe]
postgres: jelte regression [local] idle(BackendMain+0x51)[0x5ff6eb1da61b]
postgres: jelte regression [local] idle(postmaster_child_launch+0xc7)[0x5ff6eb131860]
postgres: jelte regression [local] idle(+0x444029)[0x5ff6eb136029]
postgres: jelte regression [local] idle(+0x4442ac)[0x5ff6eb1362ac]
postgres: jelte regression [local] idle(BackgroundWorkerInitializeConnection+0x0)[0x5ff6eb137920]
postgres: jelte regression [local] idle(main+0x20e)[0x5ff6eb053282]
/lib/x86_64-linux-gnu/libc.so.6(+0x2a1ca)[0x7c5b32e2a1ca]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x8b)[0x7c5b32e2a28b]
postgres: jelte regression [local] idle(_start+0x25)[0x5ff6eadcfef5]

Reverting to investigate