refactor: Remove to remove unneeded mutability (starship#577)

Removed a couple of unneeded `mut`s that were pointed out by `cargo
check`.

src/modules/git_branch.rs

@@ -9,7 +9,7 @@ use crate::configs::git_branch::GitBranchConfig;
 /// Will display the branch name if the current directory is a git repo
 pub fn module<'a>(context: &'a Context) -> Option<Module<'a>> {
     let mut module = context.new_module("git_branch");
-    let mut config = GitBranchConfig::try_load(module.config);
+    let config = GitBranchConfig::try_load(module.config);
     module.set_style(config.style);
 
     module.get_prefix().set_value("on ");

src/print.rs

@@ -47,7 +47,7 @@ pub fn get_prompt(context: Context) -> String {
         .collect::<Vec<Module>>(); // Remove segments set to `None`
 
     let mut print_without_prefix = true;
-    let mut printable = modules.iter();
+    let printable = modules.iter();
 
     for module in printable {
         // Skip printing the prefix of a module after the line_break