MDL-45336 qtype_numerical: use own strings instead of mod_quiz

These strings have already been copied from mod_quiz to qtype_numerical but
in the qtype_numerical plugin code the translations from mod_quiz were used.
Even though the strings were already present we copy the translations.

AMOS BEGIN
 CPY [invalidnumericanswer,mod_quiz],[invalidnumericanswer,qtype_numerical]
 CPY [multiplier,mod_quiz],[qtype_numerical]
 CPY [invalidnumerictolerance,mod_quiz],[qtype_numerical]
AMOS END
MDL-45336 question: move the missing type strings to question

AMOS BEGIN
 CPY [deletedquestiontext,qtype_missingtype],[deletedquestiontext,core_question]
 CPY [warningmissingtype,qtype_missingtype],[warningmissingtype,core_question]
AMOS END
MDL-45336 questionbank: use core strings instead of quiz

AMOS BEGIN
 CPY [categories,mod_quiz],[categories,question]
 CPY [unusedcategorydeleted,mod_quiz],[unusedcategorydeleted,question]
AMOS END