Fix variable marginalization for IncrementalFixedLagSmoother #3161

Job	Run time
ubuntu-clang-deprecated Debug	22m 23s
ubuntu-clang-deprecated Release	22m 54s
ubuntu-clang-quaternions Debug	22m 30s
ubuntu-clang-quaternions Release	22m 52s
ubuntu-clang-tbb Debug	23m 56s
ubuntu-clang-tbb Release	24m 30s
ubuntu-clang-cayleymap Debug	22m 11s
ubuntu-clang-cayleymap Release	22m 27s
ubuntu-clang-system-libs Debug	21m 41s
ubuntu-clang-system-libs Release	22m 55s
ubuntu-no-boost Debug	22m 1s
ubuntu-no-boost Release	22m 51s
ubuntu-no-unstable Debug	17m 50s
ubuntu-no-unstable Release	19m 0s
	5h 10m 1s

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