pgsql: Revert strlen -> strnlen optimization pre-v11.

Revert strlen -> strnlen optimization pre-v11.

We don't have a src/port substitute for that function in older branches,
so it fails on platforms lacking the function natively.  Per buildfarm.

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d5e9e2330e3a4d06b6686586fde6ea332c95be61

Modified Files
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src/backend/utils/adt/xml.c | 11 ++---------
1 file changed, 2 insertions(+), 9 deletions(-)