Remove icu backend from libxml2

[henderkes]

What does this PR do?

removes the icu backend from libxml2, because it is not recommended and will not be used if iconv is present, which we have as a depencency

this lead to roughly 32mb (before upx) of useless icu data floating around in the php binary when libxml2 was built, but intl wasn't

Checklist before merging

If your PR involves the changes mentioned below and completed the action, please tick the corresponding option.
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• If you modified *.php or *.json, run them locally to ensure your changes are valid:
  • composer cs-fix
  • composer analyse
  • composer test
  • bin/spc dev:sort-config
• If it's an extension or dependency update, please ensure the following:
  • Add your test combination to src/globals/test-extensions.php.
  • If adding new or fixing bugs, add commit message containing extension test or test extensions to trigger full test suite.

[henderkes]

This fixes errors in the gettext test when the en_US.utf-8 locale is not available on the system.

[15:27:54] [D] testing ext: gettext
[15:27:54] [I] [EXEC] /opt/static-php-cli/buildroot/bin/php -n -d "extension_dir=/opt/static-php-cli/buildroot/modules" -d "extension=gettext" --ri "gettext"
[15:27:54] [I] [EXEC] /opt/static-php-cli/buildroot/bin/php -n -d "extension_dir=/opt/static-php-cli/buildroot/modules" -d "    \$mo = '3hIElQAAAAACAAAAHAAAACwAAAAFAAAAPAAAAAAAAABQAAAABgAAAFEAAAAXAQAAWAAAAAcAAABwAQAAAQAAAAAAAAAAAAAAAgAAAAAAAAAA56S65L6LAFByb2plY3QtSWQtVmVyc2lvbjogUEFDS0FHRSBWRVJTSU9OClJlcG9ydC1Nc2dpZC1CdWdzLVRvOiAKUE8tUmV2aXNpb24tRGF0ZTogWUVBUi1NTy1EQSBITzpNSStaT05FCkxhc3QtVHJhbnNsYXRvcjogRlVMTCBOQU1FIDxFTUFJTEBBRERSRVNTPgpMYW5ndWFnZS1UZWFtOiBMQU5HVUFHRSA8TExAbGkub3JnPgpMYW5ndWFnZTogCk1JTUUtVmVyc2lvbjogMS4wCkNvbnRlbnQtVHlwZTogdGV4dC9wbGFpbjsgY2hhcnNldD1VVEYtOApDb250ZW50LVRyYW5zZmVyLUVuY29kaW5nOiA4assert(gettext(json_decode('\"\u793a\u4f8b\"', true)) === 'Example');"e(\$mo));
⚠ Validation failed: extension gettext failed sanity check. Code: 255, output:
Fatal error: Uncaught AssertionError: assert(setlocale(LC_ALL, 'en_US.utf-8') === 'en_US.utf-8') in Command line code:13
Stack trace:
#0 Command line code(13): assert(false, 'assert(setlocal...')
#1 {main}
  thrown in Command line code on line 13
----------------------------------------

Failed module: Builder for Linux
Failed validation module: Extension gettext function check