cmake: fix error in exception_tracer/CMakeLists.txt

Summary:
D7479058 changed the exception_tracer code to export their targets in the main
folly-targets.cmake file, but I forgot to remove the `install()` command
installing the old `folly-exception-tracer-targets.cmake` file.

This would cause `cmake configure` to exit with an error if it detected it
could built the exception_tracer code, but it would still generate all of the
necessary Makefiles correctly, so subsequent `make` commands would still work.

Reviewed By: yfeldblum

Differential Revision: D7626710

fbshipit-source-id: f015032c3c42cf2911fcaf9b355fc97616ff20e1

folly/experimental/exception_tracer/CMakeLists.txt

@@ -63,10 +63,4 @@ if (FOLLY_USE_SYMBOLIZER)
     LIBRARY DESTINATION ${LIB_INSTALL_DIR}
     ARCHIVE DESTINATION ${LIB_INSTALL_DIR}
   )
-  install(
-    EXPORT folly_exception_tracer
-    DESTINATION ${CMAKE_INSTALL_DIR}
-    NAMESPACE Folly::
-    FILE folly-exception-tracer-targets.cmake
-  )
 endif()