Delete unnecessary pre-header event

src/bootstrap-table.js

@@ -1848,7 +1848,6 @@ class BootstrapTable {
   }
 
   fitHeader () {
-    this.trigger('pre-header')
     if (this.$el.is(':hidden')) {
       this.timeoutId_ = setTimeout(() => this.fitHeader(), 100)
       return

src/constants/index.js

@@ -361,9 +361,6 @@ const DEFAULTS = {
   onPostBody () {
     return false
   },
-  onPreHeader () {
-    return false
-  },
   onPostHeader () {
     return false
   },
@@ -549,7 +546,6 @@ const EVENTS = {
   'toggle.bs.table': 'onToggle',
   'pre-body.bs.table': 'onPreBody',
   'post-body.bs.table': 'onPostBody',
-  'pre-header.bs.table': 'onPreHeader',
   'post-header.bs.table': 'onPostHeader',
   'post-footer.bs.table': 'onPostFooter',
   'expand-row.bs.table': 'onExpandRow',