Werk #11032: Fixed consideration of local 'old style' perfometer customizations

If a perfometer definition like:

perfometers["check_mk-mem.linux"] = my_perfometer

is set in '~/local/share/check_mk/web/plugins/perfometer" and the key is already present within the builtin perfometers, the local definition had no effect.

From now on, such local customizations are taken into account.

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