cmake: Remove redundant atlas_install_headers
Fixes a common problem in some of our CMakeLists.txt files, e.g.:
atlas_add_library( MyPackageLib
MyPackage/*.h
PUBLIC_HEADERS MyPackage
atlas_install_headers( MyPackage )
The atlas_install_headers
is redundant in this case as the library already provides the public headers (and installs them).
This MR removes all such occurrences.
cc @akraszna