Fix _remove to catch EnvCommandError instead of CalledProcessError (#10740)

run_pip() wraps CalledProcessError inside EnvCommandError, so the
except CalledProcessError handler in _remove() was dead code.  If
pip exited non-zero with 'not installed' in its output, the
EnvCommandError would propagate uncaught to _execute_operation,
which treats it as a fatal error and aborts the entire installation.

This matters when removing a package that was already uninstalled
externally — e.g. via pip directly — while Poetry is performing
an update.