Fix internal extract(timezone_minute) formulas

Through various refactorings over time, the extract(timezone_minute
from time with time zone) and extract(timezone_minute from timestamp
with time zone) implementations ended up with two different but
equally nonsensical formulas by using SECS_PER_MINUTE and
MINS_PER_HOUR interchangeably.  Since those two are of course both the
same number, the formulas do work, but for readability, fix them to be
semantically correct.

src/backend/utils/adt/date.c

@@ -2726,7 +2726,7 @@ timetz_part(PG_FUNCTION_ARGS)
 			case DTK_TZ_MINUTE:
 				result = -tz;
 				result /= SECS_PER_MINUTE;
-				FMODULO(result, dummy, (double) SECS_PER_MINUTE);
+				FMODULO(result, dummy, (double) MINS_PER_HOUR);
 				break;
 
 			case DTK_TZ_HOUR:

src/backend/utils/adt/timestamp.c

@@ -4844,7 +4844,7 @@ timestamptz_part(PG_FUNCTION_ARGS)
 
 			case DTK_TZ_MINUTE:
 				result = -tz;
-				result /= MINS_PER_HOUR;
+				result /= SECS_PER_MINUTE;
 				FMODULO(result, dummy, (double) MINS_PER_HOUR);
 				break;