box: remove unused FIELD_TYPE_MAX

Patch aa549401 "Split key_def.h/.cc" accidentally added
FIELD_TYPE_MAP member to `enum field_type`. Currently this
enum is only used to define index parts. We don't support
'map' indexed field type at least in 1.7.x.

See #2652

src/box/field_def.c

@@ -40,7 +40,6 @@ const char *field_type_strs[] = {
 	/* [FIELD_TYPE_NUMBER]   = */ "number",
 	/* [FIELD_TYPE_INTEGER]  = */ "integer",
 	/* [FIELD_TYPE_SCALAR]   = */ "scalar",
-	/* [FIELD_TYPE_MAP]      = */ "map",
 };
 
 enum field_type

src/box/field_def.h

@@ -50,7 +50,6 @@ enum field_type {
 	FIELD_TYPE_NUMBER,
 	FIELD_TYPE_INTEGER,
 	FIELD_TYPE_SCALAR,
-	FIELD_TYPE_MAP,
 	field_type_MAX
 };