Fix compiling with -fno-common

GCC 10 will enable -fno-common by default[0], which causes the linker to fail like this [1]:

`ld: pinfo-colors.o:(.bss+0x34): multiple definition of `use_manual'; pinfo-pinfo.o:(.bss+0x10): first defined here`

Fix this by removing the extraneous definition from `parse_config.h`.

[0] https://gcc.gnu.org/gcc-10/porting_to.html#common
[1] https://bugs.gentoo.org/706548

src/parse_config.h

@@ -85,8 +85,6 @@ typedef struct colours
 colours;
 #endif /* HAVE_CURSES_COLOR */
 
-int use_manual;
-
 int parse_config (void);
 int parse_line (char *line);
 char *str_toupper (char *s);