[BUGFIX] Remove hardcoded limits in T3D export

The memory_limit is hardcoded to 256m and
max_execution_time to 600.

There is no reason to hardcode those settings,
sometimes a bit more of both is needed.

Change-Id: I3e690eea1a079ed6cadd548230afc405e005b115
Resolves: #27272
Resolves: #17020
Resolves: #17045
Resolves: #18926
Releases: 4.5, 4.6
Reviewed-on: http://review.typo3.org/2610
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Kay Strobach
Tested-by: Kay Strobach
Reviewed-by: Björn Pedersen
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert