theme/powerline-plain: remove copy of powerline.base

It seems like the copied functions were just slightly out of date without any altered functionality.

The one actual difference was `$POWERLINE_PROMPT_AFTER` which is blank by default and is not documented anywhere and is not used anywhere and...I will guess it's not used.

This theme seems to be about not using the powerline font, as opposed to being "simpler" in some way.

docs/themes-list/powerline-plain.rst

@@ -4,11 +4,6 @@ Powerline Plain Theme
 =====================
 
 A colorful theme, where shows a lot information about your shell session.
-The plain powerline theme provides a simpler shell with less information.
+The plain powerline theme does not require the installation of the powerline font.
 
 See :ref:`powerline_base` for general information about the powerline theme.
-
-Soft Separators
-^^^^^^^^^^^^^^^
-
-Adjacent segments having the same background color will use a less-pronouced (i.e. soft) separator between them.

themes/powerline-plain/powerline-plain.theme.bash

@@ -1,6 +1,7 @@
-#!/usr/bin/env bash
-
-. "$BASH_IT/themes/powerline-plain/powerline-plain.base.bash"
+# shellcheck shell=bash
+# shellcheck disable=SC2034 # Expected behavior for themes.
+# shellcheck source-path=SCRIPTDIR/../powerline
+source "${BASH_IT?}/themes/powerline/powerline.base.bash"
 
 USER_INFO_SSH_CHAR=${POWERLINE_USER_INFO_SSH_CHAR:="⌁ "}
 USER_INFO_THEME_PROMPT_COLOR=${POWERLINE_USER_INFO_COLOR:=32}
@@ -12,7 +13,6 @@ POWERLINE_COMPACT_AFTER_SEPARATOR=${POWERLINE_COMPACT_AFTER_SEPARATOR:=${POWERLI
 POWERLINE_COMPACT_BEFOR_FIRST_SEGMENT=${POWERLINE_COMPACT_BEFORE_FIRST_SEGMENT:=${POWERLINE_COMPACT}}
 POWERLINE_COMPACT_AFTER_LAST_SEGMENT=${POWERLINE_COMPACT_AFTER_LAST_SEGMENT:=${POWERLINE_COMPACT}}
 POWERLINE_COMPACT_PROMPT=${POWERLINE_COMPACT_PROMPT:=${POWERLINE_COMPACT}}
-POWERLINE_PROMPT_AFTER=${POWERLINE_PROMPT_AFTER:-""}
 
 PYTHON_VENV_CHAR=${POWERLINE_PYTHON_VENV_CHAR:="ⓔ "}
 CONDA_PYTHON_VENV_CHAR=${POWERLINE_CONDA_PYTHON_VENV_CHAR:="ⓔ "}