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/company_base.h

@@ -170,7 +170,6 @@ struct Company : CompanyProperties, CompanyPool::PoolItem<&_company_pool> {
 Money CalculateCompanyValue(const Company *c, bool including_loan = true);
 Money CalculateCompanyValueExcludingShares(const Company *c, bool including_loan = true);
 
-extern uint _next_competitor_start;
 extern uint _cur_company_tick_index;
 
 #endif /* COMPANY_BASE_H */