pgsql: Fix stale comment about sample_frac adjustment

Fix stale comment about sample_frac adjustment

A comment was left behind referencing sample rate adjustment removed
from 8ad51b5f44. So clean that up. While at it also remove the sample
rate clamping which should not be necessary without the clamping, and
just check that with an assert.

Reported-by: Tom Lane
Discussion: https://postgr.es/m/951485.1672461744%40sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/211d80c065626d1a9188188d78ede85d799b93b1

Modified Files
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contrib/postgres_fdw/postgres_fdw.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)