remove `alpaka::onHost::example::enabledExecutors`

[psychocoderHPC]

With #395 alpaka::onHost::example::enabledExecutors was deprecated.
The variable should be removed before the release 3.0.0 and

alpaka3/scripts/create-single-header.sh

Line 8 in 0a9e7e3

$tmp_dir/clone/amalgamate -o $tmp_dir -v -a -n 'alpaka' -I $workingcopy_dir/include -- $workingcopy_dir/include/alpaka/alpaka.hpp $workingcopy_dir/include/alpaka/onHost/example/executors.hpp

must be adjusted.

• remove $workingcopy_dir/include/alpaka/onHost/example/executors.hpp