Revert "Update pgvector to v0.6.0, third attempt" (#6610)

The issue is still unsolved because of shmem size in VMs. Need to figure it out before applying this patch.

For more details:

```
ERROR:  could not resize shared memory segment "/PostgreSQL.2892504480" to 16774205952 bytes: No space left on device
```

As an example, the same issue in community pgvector/pgvector#453.

.dockerignore

@@ -17,7 +17,6 @@
 !libs/
 !neon_local/
 !pageserver/
-!patches/
 !pgxn/
 !proxy/
 !s3_scrubber/

Dockerfile.compute-node

@@ -241,12 +241,9 @@ RUN wget https://github.com/df7cb/postgresql-unit/archive/refs/tags/7.7.tar.gz -
 FROM build-deps AS vector-pg-build
 COPY --from=pg-build /usr/local/pgsql/ /usr/local/pgsql/
 
-COPY patches/pgvector.patch /pgvector.patch
-
-RUN wget https://github.com/pgvector/pgvector/archive/refs/tags/v0.6.0.tar.gz -O pgvector.tar.gz && \
-    echo "b0cf4ba1ab016335ac8fb1cada0d2106235889a194fffeece217c5bda90b2f19 pgvector.tar.gz" | sha256sum --check && \
+RUN wget https://github.com/pgvector/pgvector/archive/refs/tags/v0.5.1.tar.gz -O pgvector.tar.gz && \
+    echo "cc7a8e034a96e30a819911ac79d32f6bc47bdd1aa2de4d7d4904e26b83209dc8 pgvector.tar.gz" | sha256sum --check && \
     mkdir pgvector-src && cd pgvector-src && tar xvzf ../pgvector.tar.gz --strip-components=1 -C . && \
-    patch -p1 < /pgvector.patch && \
     make -j $(getconf _NPROCESSORS_ONLN) PG_CONFIG=/usr/local/pgsql/bin/pg_config && \
     make -j $(getconf _NPROCESSORS_ONLN) install PG_CONFIG=/usr/local/pgsql/bin/pg_config && \
     echo 'trusted = true' >> /usr/local/pgsql/share/extension/vector.control

pgxn/neon/pagestore_smgr.c

@@ -45,7 +45,6 @@
  */
 #include "postgres.h"
 
-#include "access/parallel.h"
 #include "access/xact.h"
 #include "access/xlog.h"
 #include "access/xlogdefs.h"
@@ -2713,14 +2712,10 @@ neon_start_unlogged_build(SMgrRelation reln)
 	reln->smgr_relpersistence = RELPERSISTENCE_UNLOGGED;
 
 	/*
-	 * Create the local file. In a parallel build, the leader is expected to
-	 * call this first and do it.
-	 *
 	 * FIXME: should we pass isRedo true to create the tablespace dir if it
 	 * doesn't exist? Is it needed?
 	 */
-	if (!IsParallelWorker())
-		mdcreate(reln, MAIN_FORKNUM, false);
+	mdcreate(reln, MAIN_FORKNUM, false);
 }
 
 /*
@@ -2744,17 +2739,7 @@ neon_finish_unlogged_build_phase_1(SMgrRelation reln)
 	Assert(unlogged_build_phase == UNLOGGED_BUILD_PHASE_1);
 	Assert(reln->smgr_relpersistence == RELPERSISTENCE_UNLOGGED);
 
-	/*
-	 * In a parallel build, (only) the leader process performs the 2nd
-	 * phase.
-	 */
-	if (IsParallelWorker())
-	{
-		unlogged_build_rel = NULL;
-		unlogged_build_phase = UNLOGGED_BUILD_NOT_IN_PROGRESS;
-	}
-	else
-		unlogged_build_phase = UNLOGGED_BUILD_PHASE_2;
+	unlogged_build_phase = UNLOGGED_BUILD_PHASE_2;
 }
 
 /*