hadoop-common's JNIFlags.cmake is redundant and can be removed

With the integration of:

• HADOOP-12036 Consolidate all of the cmake extensions in one *directory
• HADOOP-12104 Migrate Hadoop Pipes native build to new CMake
• HDFS-8635 Migrate HDFS native build to new CMake framework
• MAPREDUCE-6407 Migrate MAPREDUCE native build to new CMake
• YARN-3827 Migrate YARN native build to new CMake framework

hadoop-common-project/hadoop-common/src/JNIFlags.cmake is now redundant and can be removed