config: Rename the `delete` callback name

`delete` is a reserved keyword in C++.

include/git2/config.h

@@ -30,7 +30,7 @@ struct git_config_file {
 	int (*open)(struct git_config_file *);
 	int (*get)(struct git_config_file *, const char *key, const char **value);
 	int (*set)(struct git_config_file *, const char *key, const char *value);
-	int (*delete)(struct git_config_file *, const char *key);
+	int (*del)(struct git_config_file *, const char *key);
 	int (*foreach)(struct git_config_file *, int (*fn)(const char *, const char *, void *), void *data);
 	void (*free)(struct git_config_file *);
 };

src/config.c

@@ -171,7 +171,7 @@ int git_config_delete(git_config *cfg, const char *name)
 	internal = git_vector_get(&cfg->files, 0);
 	file = internal->file;
 
-	return file->delete(file, name);
+	return file->del(file, name);
 }
 
 /**************

src/config_file.c

@@ -452,7 +452,7 @@ int git_config_file__ondisk(git_config_file **out, const char *path)
 	backend->parent.open = config_open;
 	backend->parent.get = config_get;
 	backend->parent.set = config_set;
-	backend->parent.delete = config_delete;
+	backend->parent.del = config_delete;
 	backend->parent.foreach = file_foreach;
 	backend->parent.free = backend_free;