Remove interface linking for PRIVATE linkage

[maxtropets]

Kind of a follow up to #797

Even PRIVATE linkage retains

# Create imported target snmalloc::snmallocshim-static
add_library(snmalloc::snmallocshim-static STATIC IMPORTED)

set_target_properties(snmalloc::snmallocshim-static PROPERTIES
  INTERFACE_LINK_LIBRARIES "\$<LINK_ONLY:snmalloc::snmalloc>"
)

in the resulting cmake config.

https://cmake.org/cmake/help/latest/command/target_link_libraries.html#libraries-for-both-a-target-and-its-dependents says that

"Library dependencies are transitive by default with this signature. When this target is linked into another target then the libraries linked to this target will appear on the link line for the other target too. This transitive "link interface" is stored in the [INTERFACE_LINK_LIBRARIES](https://cmake.org/cmake/help/latest/prop_tgt/INTERFACE_LINK_LIBRARIES.html#prop_tgt:INTERFACE_LINK_LIBRARIES) target property and may be overridden by setting the property directly."

Therefore we unset the variable, although it doesn't look right, maybe.

[mjp41]

LGTM, minor comment addition would be nice.