Retire findoidjoins.

In the wake of commit 62f3409, we no longer need this tool.

Discussion: https://postgr.es/m/3240355.1612129197@sss.pgh.pa.us

Author: tglsfdc

src/tools/RELEASE_CHANGES

@@ -80,8 +80,6 @@ but there may be reasons to do them at other times as well.
 * Update Unicode data: Edit UNICODE_VERSION and CLDR_VERSION in
   src/Makefile.global.in, run make update-unicode, and commit.
 
-* Update the oidjoins regression test (see src/tools/findoidjoins/).
-
 
 Starting a New Development Cycle
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