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