Remove method pid

It does nothing, there are no tests, and there is a rakudo PR to
remove it completely.

Author: AlexDaniel

doc/Type/Proc.pod6

@@ -141,10 +141,4 @@ Returns the exit code of the external process, or -1 if it has not exited yet.
 Returns the signal number with which the external process was killed, or 0 or
 an undefined value otherwise.
 
-=head2 method pid
-
-    method pid(Proc:D:)
-
-Returns the Process Identifier (pid) of the external process, if available.
-
 =end pod