[gdb/testsuite] Fix gdb.ada/mi_catch_ex{,_hand}.exp FAILs

The test-case gdb.ada/mi_catch_ex.exp tests two scenarios.

The two scenarios are separated by:
- a mi_run_to_main (before commit b75d55d4 "Eliminate mi_run_to_main,
  introduce mi_clean_restart")
- a mi_runto_main (after that commit)

The now removed mi_run_to_main contained a mi_delete_breakpoints, while
mi_runto_main does not, which causes FAILs in the second scenario.

Fix this by adding the missing mi_delete_breakpoints in the test-case, and
likewise in gdb.ada/mi_catch_ex_hand.exp.

Reg-tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-10-14  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26732
	* gdb.ada/mi_catch_ex.exp: Do mi_delete_breakpoints before running
	scenario 2.
	* gdb.ada/mi_catch_ex_hand.exp: Same.