Remove develop flag StrictSafepointChecks

There doesn't seem to be a reason to turn off these checks.

One reason may have been to have a quick return for product:

- debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
+ _thread->check_for_valid_safepoint_state(false);

But, check_for_valid-safepoint_state is declared in thread.hpp as:

  void check_for_valid_safepoint_state(bool potential_vm_operation) PRODUCT_RETURN;


So doesn't require debug_only or a const false flag to not generate a call in production.