Remove manually created symbolic link to ninja-build (#18437) (#18456)

Author: nickguletskii

ci/docker/install/centos7_core.sh

@@ -40,9 +40,6 @@ yum -y install wget
 yum -y install unzip
 yum -y install ninja-build
 
-# Centos 7 only provides ninja-build
-ln -s /usr/bin/ninja-build /usr/bin/ninja
-
 # CMake 3.13.2+ is required
 mkdir /opt/cmake && cd /opt/cmake
 wget -nv https://cmake.org/files/v3.13/cmake-3.13.5-Linux-x86_64.sh