This was added in commit ebd1b95bb5f9235d1dba1b840fd746c9b53320d2 (INTEGRATION: CWS xmlsec08 (1.1.2); FILE ADDED; 2005-03-10) without any real commit message to explain why this is necessary.
system-xmlsec (not containing this patch) works fine for our XML signing purposes with software certificates, and just recently I learned that even hardware-based certificates work fine without this patch, so it can go away.
I assume this was a refactor to allow some new feature as a next step, but that feature was never implemented.
Change-Id: I31639230483cd34b14b35fd41b4fcd8284476138 Reviewed-on: https://gerrit.libreoffice.org/55296
f06004c03bbd libxmlsec: drop not needed xmlsec1-customkeymanage.patch.1
external/xmlsec/UnpackedTarball_xmlsec.mk | 1 -
external/xmlsec/xmlsec1-customkeymanage.patch.1 | 4321 --------------------
.../xmlsec/nss/securityenvironment_nssimpl.cxx | 36 -
3 files changed, 4358 deletions(-)