drop unused functions and configure checks

Remove the following C++ functions/methods, which have no callers:

common.cpp:
- read_loop
- narrow_string_safe
- escape_string_for_double_quotes
- read_unquoted_escape
- format_size
- format_size_safe
- valid_func_name
- get_executable_path

env.cpp:
- env_stack_t::set_empty
- env_stack_t::set_argv

fallback.cpp:
- fish_mkstemp_cloexec
- flock

proc_util.cpp:
- parse_util_slice_length
- parse_util_argument_is_help

path.cpp:
- path_get_path
- path_as_implicit_cd
- path_apply_working_directory
- path_emit_config_directory_messages
- path_get_data_remoteness
- path_get_config_remoteness
- path_is_valid
- paths_are_same_file

wcstringutil.cpp:
- split_string_tok

wutil.cpp:
- wgetcwd
- wunlink
- wrealpath
- wrename
- file_id_for_path
- fish_wcstoull

Also drop unused configure checks/defines:
- HAVE_FLOCK

doc_src/license.rst

@@ -231,36 +231,6 @@ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH RE
 
 ----
 
-**License for flock**
-
-``fish`` also contains small amounts of code from NetBSD, namely the ``flock`` fallback function. This code is copyright 2001 The NetBSD Foundation, Inc., and derived from software contributed to The NetBSD Foundation by Todd Vierling.
-
-The NetBSD license follows.
-
-Redistribution and use in source and binary forms, with or without
-modification, are permitted provided that the following conditions are met:
-
-1. Redistributions of source code must retain the above copyright
-   notice, this list of conditions and the following disclaimer.
-
-2. Redistributions in binary form must reproduce the above copyright
-   notice, this list of conditions and the following disclaimer in the
-   documentation and/or other materials provided with the distribution.
-
-THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
-"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
-TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
-PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
-BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
-CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
-SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
-INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
-CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
-ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
-POSSIBILITY OF SUCH DAMAGE.
-
-----
-
 **MIT License**
 
 ``fish`` includes a copy of Alpine.js, which is copyright 2019-2021 Caleb Porzio and contributors, and licensed under the MIT License. It also includes the Dracula theme, which is copyright 2018 Dracula Team, and is licensed under the same license.