Remove doctests action (#5556)

We're no longer using `cargo nextest` for our main test job. The separate action for doctests was only necessary because `cargo nextest` does not support doctests, it can be removed.

Hoping this will result in less data cached but we'll see.

.github/workflows/ci.yml

@@ -82,9 +82,7 @@ jobs:
 
       - uses: Swatinem/rust-cache@v1
         with:
-          key: ${{ matrix.style }}v2 # increment this to bust the cache if needed
-
-      # - uses: taiki-e/install-action@nextest
+          key: ${{ matrix.style }}v3 # increment this to bust the cache if needed
 
       - name: Tests
         uses: actions-rs/cargo@v1
@@ -92,12 +90,6 @@ jobs:
           command: test
           args: --workspace --profile ci --exclude nu_plugin_* ${{ matrix.flags }}
 
-      - name: Doctests
-        uses: actions-rs/cargo@v1
-        with:
-          command: test
-          args: --workspace --profile ci --exclude nu_plugin_* --doc ${{ matrix.flags }}
-
   python-virtualenv:
     env:
       NUSHELL_CARGO_TARGET: ci