Change: replace per-AI "start_date" with a global "competitors_interval" (#10653)

The per-AI "start_date" is a lot of custom code, and was rarely
used in the way it was meant.

While at it, also ported this part over to the new timer system.

src/table/settings/difficulty_settings.ini

@@ -57,6 +57,15 @@ interval = 1
 post_cb  = MaxNoAIsChange
 cat      = SC_BASIC
 
+[SDT_VAR]
+var      = difficulty.competitors_interval
+type     = SLE_UINT16
+from     = SLV_AI_START_DATE
+def      = 10
+min      = MIN_COMPETITORS_INTERVAL
+max      = MAX_COMPETITORS_INTERVAL
+interval = 1
+
 [SDT_VAR]
 var      = difficulty.competitor_start_time
 type     = SLE_UINT8