Rename spawn-polkit-agent.c to just polkit-agent.c

The functions in the file were *not* using the spawn prefix,
and the prefix seems completely unnecessary.

src/machine/machinectl.c

@@ -47,6 +47,7 @@
 #include "parse-argument.h"
 #include "parse-util.h"
 #include "path-util.h"
+#include "polkit-agent.h"
 #include "pretty-print.h"
 #include "process-util.h"
 #include "ptyfwd.h"
@@ -55,7 +56,6 @@
 #include "signal-util.h"
 #include "sort-util.h"
 #include "spawn-ask-password-agent.h"
-#include "spawn-polkit-agent.h"
 #include "stdio-util.h"
 #include "string-table.h"
 #include "strv.h"