pgsql: Remove git-external-diff from tools directory

Remove git-external-diff from tools directory

The git-external-diff bash script could not be executed due to syntax
errors without it being reported, which indicates that it hasn't been
used by anyone in a long time. Rather than spending energy on fixing
dead code, the conclusion in the thread was to remove it.

Reported-by: zmuxuny <xiahoumuxuan(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAMPOERnKK-vNtc_Ooddb9WGbDepGi1=pxOFfiGrb4_Cqzyu69w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c0f2b6dc82718f17a2190f474450885df6c06402

Modified Files
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src/tools/git-external-diff | 59 ---------------------------------------------
1 file changed, 59 deletions(-)