pgsql: Revert "Add support for parsing of large XML data (>= 10MB)"

Revert "Add support for parsing of large XML data (>= 10MB)"

This reverts commit 2197d06224a1, following a discussion over a Coverity
report where issues like the "Billion laugh attack" could cause the
backend to waste CPU and memory even if a client applied checks on the
size of the data given in input, and libxml2 does not offer guarantees
that input limits are respected under XML_PARSE_HUGE.

Discussion: https://postgr.es/m/ZbHlgrPLtBZyr_QW@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f2743a7d70e7b2891277632121bb51e739743a47

Modified Files
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contrib/xml2/xpath.c        |  4 ++--
contrib/xml2/xslt_proc.c    |  4 ++--
src/backend/utils/adt/xml.c | 38 +++++++++-----------------------------
3 files changed, 13 insertions(+), 33 deletions(-)