vcmi: convert magic schools to MAGIC_SCHOOL_SKILL

It is already used by battlefields.

config/skills.json

@@ -410,8 +410,8 @@
 		"base" : {
 			"effects" : {
 				"main" : {
-					"subtype" : "skill.fireMagic",
-					"type" : "SECONDARY_SKILL_PREMY",
+					"subtype" : 2,
+					"type" : "MAGIC_SCHOOL_SKILL",
 					"valueType" : "BASE_NUMBER"
 				}
 			}
@@ -438,8 +438,8 @@
 		"base" : {
 			"effects" : {
 				"main" : {
-					"subtype" : "skill.airMagic",
-					"type" : "SECONDARY_SKILL_PREMY",
+					"subtype" : 1,
+					"type" : "MAGIC_SCHOOL_SKILL",
 					"valueType" : "BASE_NUMBER"
 				}
 			}
@@ -466,8 +466,8 @@
 		"base" : {
 			"effects" : {
 				"main" : {
-					"subtype" : "skill.waterMagic",
-					"type" : "SECONDARY_SKILL_PREMY",
+					"subtype" : 4,
+					"type" : "MAGIC_SCHOOL_SKILL",
 					"valueType" : "BASE_NUMBER"
 				}
 			}
@@ -494,8 +494,8 @@
 		"base" : {
 			"effects" : {
 				"main" : {
-					"subtype" : "skill.earthMagic",
-					"type" : "SECONDARY_SKILL_PREMY",
+					"subtype" : 8,
+					"type" : "MAGIC_SCHOOL_SKILL",
 					"valueType" : "BASE_NUMBER"
 				}
 			}

lib/mapObjects/CGHeroInstance.cpp

@@ -595,9 +595,7 @@ int32_t CGHeroInstance::getSpellSchoolLevel(const spells::Spell * spell, int32_t
 
 	spell->forEachSchool([&, this](const spells::SchoolInfo & cnf, bool & stop)
 	{
-		int32_t thisSchool = std::max<int32_t>(
-			valOfBonuses(Bonus::SECONDARY_SKILL_PREMY, cnf.skill),
-			valOfBonuses(Bonus::MAGIC_SCHOOL_SKILL, 1 << (static_cast<ui8>(cnf.id)))); //FIXME: Bonus shouldn't be additive (Witchking Artifacts : Crown of Skies)
+		int32_t thisSchool = valOfBonuses(Bonus::MAGIC_SCHOOL_SKILL, 1 << (static_cast<ui8>(cnf.id))); //FIXME: Bonus shouldn't be additive (Witchking Artifacts : Crown of Skies)
 		if(thisSchool > skill)
 		{
 			skill = thisSchool;