fix test's and delete unused code

autoload/ale/fixers/mix_format.vim

@@ -9,8 +9,7 @@ endfunction
 
 function! ale#fixers#mix_format#Fix(buffer) abort
     return {
-    \   'command': ale#handlers#elixir#GetExecutable(a:buffer)
-    \       . ale#fixers#mix_format#GetExecutable(a:buffer)
+    \   'command': ale#Escape(ale#fixers#mix_format#GetExecutable(a:buffer))
     \       . ' format %t',
     \   'read_temporary_file': 1,
     \}

autoload/ale/handlers/elixir.vim

@@ -1,5 +0,0 @@
-call ale#Set('elixir_executable', '')
-
-function! ale#handlers#elixir#GetExecutable(buffer) abort
-    return ale#Var(a:buffer, 'elixir_executable')
-endfunction

test/fixers/test_elixir_fixer_callback.vader

@@ -1,16 +0,0 @@
-Before:
-  call ale#test#SetDirectory('/testplugin/test/fixers')
-
-After:
-  call ale#test#RestoreDirectory()
-
-Execute(The executable path should be correct):
-  call ale#test#SetFilename('../elixir-test-files/testfile.ex')
-
-  AssertEqual
-  \ {
-  \   'read_temporary_file': 1,
-  \   'command': 'mix format'
-  \     . ' %t',
-  \ },
-  \ ale#fixers#elixir#Fix(bufnr(''))

test/fixers/test_mix_format_fixer_callback.vader

@@ -0,0 +1,20 @@
+Before:
+  call ale#test#SetDirectory('/testplugin/test/fixers')
+  Save g:ale_elixir_mix_executable
+
+  let g:ale_elixir_mix_executable = 'xxxinvalid'
+
+After:
+  call ale#test#RestoreDirectory()
+
+Execute(The mix_format callback should return the correct default values):
+  call ale#test#SetFilename('../elixir-test-files/testfile.ex')
+
+  AssertEqual
+  \ {
+  \   'read_temporary_file': 1,
+  \   'command': ale#Escape('xxxinvalid')
+  \     . ' format %t',
+  \ },
+  \ ale#fixers#mix_format#Fix(bufnr(''))
+