Bump $wgMaxShellMemory to 512MB; drop ctag generation

phpsh fires a volley of warnings on start-up that all resolve to OOMs,
triggered by the hard quota set by $wgMaxShellMemory. This patch increases the
quota to 512MB (up from the default 128MB).

The patch also drops automatic generation of ctags. Autocompletion in phpsh
doesn't work very well and loading tags makes phpsh's slow start-up time even
slower.

Change-Id: I840f498932cccb96a580d444c08fa9bd7cabb70d