remove unused includes

This patch removes includes that are not used. The removals were found with
include-what-you-use which checks if any of the symbols from a header is
in use.

src/shared/install.c

@@ -36,7 +36,6 @@
 #include "install.h"
 #include "conf-parser.h"
 #include "conf-files.h"
-#include "specifier.h"
 #include "install-printf.h"
 #include "special.h"