[Sync-En] fix unresolved exception entity references

[lacatoire]

Sync with EN commit e94f86354b737b80a099ccb14b32c07abe6d96e5 (php/doc-en#5779).

• Reference the exception classes of gmagick, lua, mysql_xdevapi, svm, swoole, varnish and xmldiff from their book.xml
• Remove the self-referencing &Methods; / Constructor blocks and the undefined &reference.<ext>.entities.*; entities
• Delete ten orphaned reference.xml files (ev, gearman, lua, luasandbox, pthreads, quickhash, sync, wkhtmltox, xlswriter, yaconf), included by nothing

Fixes: #1065