Delete duplicated FindLibXml2.cmake file

  - cmake version 3.12 defines libxml2 module; the default path is
    /usr/share/cmake-3.<sub-version>/Modules/FindLibXml2.cmake
  - OAI requires minimum cmake version 3.16

cmake_targets/tools/MODULES/FindLibXml2.cmake

-# - Try to find the LibXml2 xml processing library
-# Once done this will define
-#
-#  LIBXML2_FOUND - System has LibXml2
-#  LIBXML2_INCLUDE_DIR - The LibXml2 include directory
-#  LIBXML2_LIBRARIES - The libraries needed to use LibXml2
-#  LIBXML2_DEFINITIONS - Compiler switches required for using LibXml2
-#  LIBXML2_XMLLINT_EXECUTABLE - The XML checking tool xmllint coming with LibXml2
-
-#=============================================================================
-# Copyright 2006-2009 Kitware, Inc.
-# Copyright 2006 Alexander Neundorf <neundorf@kde.org>
-#
-# Distributed under the OSI-approved BSD License (the "License");
-# see accompanying file Copyright.txt for details.
-#
-# This software is distributed WITHOUT ANY WARRANTY; without even the
-# implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
-# See the License for more information.
-#=============================================================================
-# (To distributed this file outside of CMake, substitute the full
-#  License text for the above reference.)
-
-# use pkg-config to get the directories and then use these values
-# in the FIND_PATH() and FIND_LIBRARY() calls
-FIND_PACKAGE(PkgConfig)
-PKG_CHECK_MODULES(PC_LIBXML libxml-2.0)
-SET(LIBXML2_DEFINITIONS ${PC_LIBXML_CFLAGS_OTHER})
-
-FIND_PATH(LIBXML2_INCLUDE_DIR NAMES libxml/xpath.h
-   HINTS
-   ${PC_LIBXML_INCLUDEDIR}
-   ${PC_LIBXML_INCLUDE_DIRS}
-   PATH_SUFFIXES libxml2
-   )
-
-FIND_LIBRARY(LIBXML2_LIBRARIES NAMES xml2 libxml2
-   HINTS
-   ${PC_LIBXML_LIBDIR}
-   ${PC_LIBXML_LIBRARY_DIRS}
-   )
-
-FIND_PROGRAM(LIBXML2_XMLLINT_EXECUTABLE xmllint)
-# for backwards compat. with KDE 4.0.x:
-SET(XMLLINT_EXECUTABLE "${LIBXML2_XMLLINT_EXECUTABLE}")
-
-# handle the QUIETLY and REQUIRED arguments and set LIBXML2_FOUND to TRUE if 
-# all listed variables are TRUE
-INCLUDE(FindPackageHandleStandardArgs)
-FIND_PACKAGE_HANDLE_STANDARD_ARGS(LibXml2 DEFAULT_MSG LIBXML2_LIBRARIES LIBXML2_INCLUDE_DIR)
-
-MARK_AS_ADVANCED(LIBXML2_INCLUDE_DIR LIBXML2_LIBRARIES LIBXML2_XMLLINT_EXECUTABLE)
-