Remove outdated _FORTIFY_SOURCE #undefs

AFAICT, all of these were added prior to the new-style FORTIFY going
in. Because clang saw two definitions of the "same" function, it got
upset. (e.g. 35d937e4)

Now that our FORTIFY functions are overloads of their library functions,
these undefs can be removed.

(This is a step toward making FORTIFY a library with -U_FORTIFY_SOURCE
on it, as requested in the bug.)

Bug: 12231437
Test: CtsBionicUnitTests. No new failures.

Change-Id: I844ff77106ee2bea72ae1cd0bb85d146939ecd05