Remove readline from bundled Python 3

To follow up #1945 (confidential) and to tackle some readline problems around GNU Readline/Editline Library (libedit, ported from NetBSD), remove and unlink readline from bundled Python 3 of omnibus-gitlab.

readline feature of Python is not required to render reST files (.rst) with gitlab-markup.

Checklist

Checking problems when linking readline libs on CPython build:

on configure

checking how to link readline libs... none
checking for rl_pre_input_hook in -lreadline... no
checking for rl_completion_display_matches_hook in -lreadline... no
checking for rl_resize_terminal in -lreadline... no
checking for rl_completion_matches in -lreadline... no
checking for append_history in -lreadline... no

=> OK (while "no"s appear)

on make

gcc -pthread -c -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall    -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration   -I. -I./Include   -fPIC -DPy_BUILD_CORE -o Parser/myreadline.o Parser/myreadline.c

=> OK

ref.

See also #2361 (closed), !3069 (merged), !3121 (closed)