Revert unset prompt_callback and extra call to git_prompt_reset

gitprompt.sh

@@ -65,7 +65,6 @@ function get_theme() {
 }
 
 function git_prompt_load_theme() {
-  git_prompt_reset
   get_theme
   local DEFAULT_THEME_FILE="${__GIT_PROMPT_DIR}/themes/Default.bgptheme"
   source "${DEFAULT_THEME_FILE}"
@@ -179,7 +178,6 @@ git_prompt_reset() {
   for var in GIT_PROMPT_DIR __GIT_PROMPT_COLORS_FILE __PROMPT_COLORS_FILE __GIT_STATUS_CMD GIT_PROMPT_THEME_NAME; do
     unset $var
   done
-  unset -f prompt_callback
 }
 
 # gp_format_exit_status RETVAL