Remove references to `--dev-only` install option

According to
https://github.com/python-poetry/poetry/issues/4864#issuecomment-1002465303:
> The `--dev-only` option was never available in a release, but was already
> available in the master branch. Because it's a feature we will probably not
> backport this to the 1.1 branch.

Given that the option was only ever present when installing Poetry from master
and was never available as part of a release, I think it's clearer to just
remove all references to it rather than include a deprecation notice that would
only be relevant to a tiny fraction of Poetry users.

docs/cli.md

@@ -167,10 +167,6 @@ It's also possible to only install specific dependency groups by using the `only
 poetry install --only test,docs
 ```
 
-{{% note %}}
-The `--dev-only` option is now deprecated. You should use the `--only dev` notation instead.
-{{% /note %}}
-
 See [Dependency groups]({{< relref "managing-dependencies#dependency-groups" >}}) for more information
 about dependency groups.
 
@@ -231,7 +227,6 @@ option is used.
 * `--extras (-E)`: Features to install (multiple values allowed).
 * `--all-extras`: Install all extra features (conflicts with --extras).
 * `--no-dev`: Do not install dev dependencies. (**Deprecated**)
-* `--dev-only`: Only install dev dependencies. (**Deprecated**)
 * `--remove-untracked`: Remove dependencies not presented in the lock file. (**Deprecated**)
 
 {{% note %}}

src/poetry/console/commands/group_command.py

@@ -88,7 +88,6 @@ class GroupCommand(EnvCommand):
             ("default", "only", MAIN_GROUP),
             ("no-dev", "only", MAIN_GROUP),
             ("dev", "with", "dev"),
-            ("dev-only", "without", MAIN_GROUP),
         ]:
             if self.io.input.has_option(opt) and self.option(opt):
                 self.line_error(

src/poetry/console/commands/install.py

@@ -19,12 +19,6 @@ class InstallCommand(InstallerCommand):
             " (<warning>Deprecated</warning>)",
         ),
         option(
-            "dev-only",
-            None,
-            "Only install the development dependencies."
-            " (<warning>Deprecated</warning>)",
-        ),
-        option(
             "sync",
             None,
             "Synchronize the environment with the locked packages and the specified"

tests/console/commands/test_install.py

@@ -89,7 +89,6 @@ def tester(
         # deprecated options
         ("--default", {MAIN_GROUP}),
         ("--no-dev", {MAIN_GROUP}),
-        ("--dev-only", {"foo", "bar", "baz", "bim"}),
     ],
 )
 @pytest.mark.parametrize("with_root", [True, False])