MDL-36534 Quiz: Group question type unit fields
MDL-36534 question editing: clean up lang strings.

There were a number of lang strings in suboptimal places.
This commit fixes that, and also removed some unused strings.

AMOS BEGIN
 MOV [addmorechoiceblanks,qtype_multichoice],[addmorechoiceblanks,question]
 MOV [youmustenteramultiplierhere,qtype_calculated],[youmustenteramultiplierhere,qtype_numerical]
AMOS END