cleanup: remove THD::query_start_timeval()

Too easy to abuse. One tends to use it even when
microseconds are not needed and will be ignored.

sql/field.cc

@@ -6796,7 +6796,12 @@ int Field_datetime::set_time()
   THD *thd= table->in_use;
   set_notnull();
   // Here we always truncate (not round), no matter what sql_mode is
-  store_datetime(Datetime(thd, thd->query_start_timeval()).trunc(decimals()));
+  if (decimals())
+    store_datetime(Datetime(thd, Timeval(thd->query_start(),
+                                         thd->query_start_sec_part())
+                            ).trunc(decimals()));
+  else
+    store_datetime(Datetime(thd, Timeval(thd->query_start(), 0)));
   return 0;
 }
 

sql/sql_class.h

@@ -3455,10 +3455,6 @@ class THD: public THD_count, /* this must be first */
   inline ulong query_start_sec_part()
   { query_start_sec_part_used=1; return start_time_sec_part; }
   MYSQL_TIME query_start_TIME();
-  Timeval query_start_timeval()
-  {
-    return Timeval(query_start(), query_start_sec_part());
-  }
   time_round_mode_t temporal_round_mode() const
   {
     return variables.sql_mode & MODE_TIME_ROUND_FRACTIONAL ?