[OpenMP][Libomptarget] Fix cmake error on remote plugin

[atmnpatel]

Summary

Requiring 3.15 causes a build breakage, I'm sure none of the contents actually require
3.15 or above.

openmp/libomptarget/plugins/remote/CMakeLists.txt

 ##===----------------------------------------------------------------------===##
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 ##===----------------------------------------------------------------------===##
 #
 # Build a plugin (client) and server for remote offloading.
 #
 ##===----------------------------------------------------------------------===#
 
-cmake_minimum_required(VERSION 3.15)
-
 if (NOT(CMAKE_SYSTEM_NAME MATCHES "Linux"))
   libomptarget_say("Not building remote offloading plugin: only support Linux hosts.")
   return()
 endif()
 
 if (NOT(LIBOMPTARGET_ENABLE_EXPERIMENTAL_REMOTE_PLUGIN))
   return()
 endif()