Fix fbkpatch override reference script too early

Summary:
Fixes order of execution bug, the service got override and was restarted before it had the file referenced in the override.
Move the override to fb_kernel_settings, as the fb_kpatch is [opensourced](https://www.internalfb.com/diff/D67226415?dst_version_fbid=576249331819024&transaction_fbid=600555725684425).

previous attempt in D67226415 (reverted in D67288917)

Differential Revision: D67290321

fbshipit-source-id: 181c9b8f3e37065047db5eb5c008a3accf1fbd53

cookbooks/fb_kpatch/recipes/default.rb

@@ -47,13 +47,3 @@
             },
           })
 end
-fb_systemd_override 'fbkpatch' do
-  unit_name 'kpatch.service'
-  content({
-            'Service' => {
-              # Empty record is required! It tells systemd to clean existing
-              # values, as it appends values from drop ins.
-              'ExecStart' => ['', '/usr/local/bin/fbkpatch load --all'],
-            },
-          })
-end