[doc] Remove command to disable push
When using the [hub](https://github.com/github/hub) command line tool it
is not possible to set non-URLs as remote:
```
$ git remote set-url --push upstream disabled
Error: repository disabled/opentitan doesn't exist
```
Removing this instruction from the user guide to prevent confusion for
users who have this tool installed.
diff --git a/doc/ug/github_notes.md b/doc/ug/github_notes.md
index 0eb3676..3b7d904 100644
--- a/doc/ug/github_notes.md
+++ b/doc/ug/github_notes.md
@@ -54,7 +54,6 @@
$ git clone https://github.com/$GITHUB_USER/opentitan.git
$ cd opentitan
$ git remote add upstream https://github.com/lowRISC/opentitan.git
-$ git remote set-url --push upstream disabled
$ git remote -v
```
