Fix duplicate test in prefix.bats due to bad merge (#1603)

The test "prefix for system in /" is duplicated in test/prefix.bats.
Both tests are completely identical. This commit removes the
duplication.

It appears that the culprit is this merge from rbenv/master, from 2016:

  cf1beda362

With the current development version of bats, this leads to the
following error when running `make test`:

  Error:
  Duplicate test name(s) in file
  "/home/travis/build/pyenv/pyenv/test/prefix.bats":
  test_prefix_for_system_in_-2f

Note that the development version is what pyenv's Makefile uses. With
the latest release of bats, the duplication only leads to a warning:

  bats warning:
  duplicate test name(s) in /src/test/prefix.bats:
  test_prefix_for_system_in_-2f

test/prefix.bats

@@ -36,18 +36,6 @@ OUT
   rm -f "${BATS_TEST_DIRNAME}/libexec/pyenv-which"
 }
 
-@test "prefix for system in /" {
-  mkdir -p "${BATS_TEST_DIRNAME}/libexec"
-  cat >"${BATS_TEST_DIRNAME}/libexec/pyenv-which" <<OUT
-#!/bin/sh
-echo /bin/python
-OUT
-  chmod +x "${BATS_TEST_DIRNAME}/libexec/pyenv-which"
-  PYENV_VERSION="system" run pyenv-prefix
-  assert_success "/"
-  rm -f "${BATS_TEST_DIRNAME}/libexec/pyenv-which"
-}
-
 @test "prefix for invalid system" {
   PATH="$(path_without python)" run pyenv-prefix system
   assert_failure "pyenv: system version not found in PATH"