Drop extraneous libc overrides

Changing symbol visibility (see #1) wasn't actually necessary. The true issue was that upstream jemalloc already overrides these symbols, so we didn't need our own definitions in the first place.

As outlined upstream:

You might say that those [upstream] declarations have je_ prefix, but when they're added to jemalloc.h, there's #define je_malloc malloc placed before those declarations. See include/jemalloc/jemalloc.sh that generates jemalloc.h. I admit - that's very confusing.

Tested against FEXConfig and Ender Lilies (with thunks).