refactor: remove duplicate showIssuesAssignedToMe command

In !244 (merged) I accidentally duplicated configuration:

{
  "command": "gl.showIssuesAssignedToMe",
  "when": "gitlab:validState"
}

This meant that the command was offered twice in the command palette:

This issue didn't have any negative effect on functionality except for showing the command twice.

Since this is an unreleased issue, I'm marking it as refactoring to avoid having a CHANGELOG entry.

I've double-checked that this was the only duplicated command:

cat package.json | jq '.contributes.menus.commandPalette[].command' | sort | uniq -c
   1 "gl.cancelEditingComment"
   1 "gl.compareCurrentBranch"
   1 "gl.copyLinkToActiveFile"
   1 "gl.createComment"
   1 "gl.createSnippet"
   1 "gl.deleteComment"
   1 "gl.insertSnippet"
   1 "gl.issueSearch"
   1 "gl.mergeRequestSearch"
   1 "gl.openActiveFile"
   1 "gl.openCreateNewIssue"
   1 "gl.openCreateNewMR"
   1 "gl.openCurrentMergeRequest"
   1 "gl.openCurrentPipeline"
   1 "gl.openProjectPage"
   1 "gl.pipelineActions"
   1 "gl.projectAdvancedSearch"
   1 "gl.refreshSidebar"
   1 "gl.resolveThread"
   1 "gl.showIssuesAssignedToMe"
   1 "gl.showMergeRequestsAssignedToMe"
   1 "gl.startEditingComment"
   1 "gl.submitCommentEdit"
   1 "gl.unresolveThread"
   1 "gl.validateCIConfig"