Remove unnecessary abort() from WalSndShutdown().

Remove unnecessary abort() from WalSndShutdown().

WalSndShutdown() previously called abort() after proc_exit(0) to
silence compiler warnings. This is no longer needed, because both
WalSndShutdown() and proc_exit() are declared pg_noreturn,
allowing the compiler to recognize that the function does not return.
Also there are already other functions, such as CheckpointerMain(),
that call proc_exit() without an abort(), and they do not produce warnings.

Therefore this abort() call in WalSndShutdown() is useless and
this commit removes it.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CAHGQGwHPX1yoixq+YB5rF4zL90TMmSEa3FpHURtqW3Jc5+=oSA@mail.gmail.com