Revert "session: do not delete on flush" #148

Job	Run time
build-test (ubuntu-18.04, gcc)	5m 42s
coverity-test	0s
coverage-test	5m 44s
build-test (ubuntu-18.04, clang)	5m 0s
master-tss-build-test (gcc)	18m 0s
build-test (ubuntu-20.04, gcc)	5m 55s
master-tss-build-test (clang)	4m 59s
build-test (ubuntu-20.04, clang)	5m 3s
build-test (fedora-30, gcc)	5m 53s
build-test (fedora-30, clang)	5m 16s
build-test (opensuse-leap, gcc)	5m 56s
build-test (opensuse-leap, clang)	5m 9s
build-test (fedora-32, gcc)	5m 54s
build-test (fedora-32, clang)	5m 7s
build-test (ubuntu-22.04, gcc)	11m 18s
build-test (ubuntu-22.04, clang)	10m 48s
	1h 45m 44s

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