[Ada] Remove dead code for overlapping actuals and prefix notation

[Pierre-Marie de Rodat]

In detection of overlapping actuals we first call Refer_Same_Object to
filter function calls that act as actual parameters. There is no need to
later special-case any function calls, e.g. in the prefix notation.

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

gcc/ada/

	* sem_warn.adb (Warn_On_Overlapping_Actuals): Remove dead code.
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