Replace bughy implementation of function 'remove_all()'.

Counterexample for old implementation:
    char str[] = "aaa";
    remove_all(str, 'a');
    printf("%s\n", str); // displays "a" if we're lucky,
                         //or crashes if we're not.

dwmblocks.c

@@ -51,13 +51,13 @@ void remove_all(char *str, char to_remove) {
 	char *read = str;
 	char *write = str;
 	while (*read) {
-		if (*read == to_remove) {
+		if (*read != to_remove) {
-			read++;
 			*write = *read;
+			++write;
 		}
-		read++;
+		++read;
-		write++;
 	}
+	*write = '\0';
 }
 
 //opens process *cmd and stores output in *output