pgsql: Simplify makefiles exporting twice enable_injection_points

Simplify makefiles exporting twice enable_injection_points

This is confusing, as it exports twice the same variable.  Oversight in
6782709df81f that has spread in more places afterwards.

Reported-by: Alvaro Herrera, Tom Lane
Discussion: https://postgr.es/m/202408201630.mn6vbohjh7hh@alvherre.pgsql
Backpatch-through: 17

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/516ff05539bb70cc378e21e5e67588ec9c959cf7

Modified Files
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src/test/modules/injection_points/Makefile | 2 +-
src/test/modules/test_misc/Makefile        | 2 +-
src/test/modules/test_slru/Makefile        | 2 +-
src/test/recovery/Makefile                 | 2 +-
4 files changed, 4 insertions(+), 4 deletions(-)