Remove stdbool.h from files which don't need it

Done by manual inspection of the output of this script:
grep -r -l -w "bool\|true\|false" * | grep 'c$\|h$' > has_bool
grep -r -l "stdbool.h" * | grep 'c$\|h$' > has_include
grep -F -x -v -f has_bool has_include

src/nvim/os/server.c

@@ -1,7 +1,6 @@
 #include <stdlib.h>
 #include <string.h>
 #include <stdint.h>
-#include <stdbool.h>
 
 #include <uv.h>