pgsql: Revert "Improve error message on snapshot import in snapmgr.c"

Revert "Improve error message on snapshot import in snapmgr.c"

This reverts commit a0d87bcd9b57, following a remark from Andres Frend
that the new error can be triggered with an incorrect SET TRANSACTION
SNAPSHOT command without being really helpful for the user as it uses
the internal file name.

Discussion: https://postgr.es/m/20230914020724.hlks7vunitvtbbz4@awork3.anarazel.de
Backpatch-through: 11

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bddbbdf99b16b4f3abaa05d6e1d8ee209efeaa7a

Modified Files
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src/backend/utils/time/snapmgr.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)