When iterating over list elements with First/Next there is no need to
check if the list is present, because First intentionally returns Empty
if list is not present and the condition of subsequent loop will not be
satisfied.

Code cleanup; semantics is unaffected.

Occurrences of the redundant pattern were found with:

  $ grep First -B 3 | less

and examining the output for the calls to Present.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_ch13.adb, exp_ch5.adb, exp_ch9.adb, exp_strm.adb,
	sem_ch10.adb, sem_ch13.adb, sem_ch5.adb, sem_ch6.adb,
	sem_ch8.adb, sem_elab.adb, sem_eval.adb, sem_prag.adb,
	sem_util.adb: Remove checks for the missing list before
	iterating with First/Next; reindent code and refill comments.