Revert "session: do not delete on flush" #147

Job	Run time
master-tss-build-test (gcc)	18m 2s
coverity-test	0s
coverage-test	5m 45s
master-tss-build-test (clang)	5m 2s
build-test (ubuntu-18.04, gcc)	5m 52s
build-test (ubuntu-18.04, clang)	5m 13s
build-test (ubuntu-20.04, gcc)	6m 2s
build-test (ubuntu-20.04, clang)	4m 51s
build-test (fedora-30, gcc)	5m 55s
build-test (fedora-30, clang)	5m 16s
build-test (opensuse-leap, gcc)	5m 59s
build-test (opensuse-leap, clang)	5m 13s
build-test (fedora-32, gcc)	5m 55s
build-test (fedora-32, clang)	5m 4s
build-test (ubuntu-22.04, gcc)	11m 27s
build-test (ubuntu-22.04, clang)	10m 42s
	1h 46m 18s

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