Native: remove unused headers.x86-64 from linux.def

`headers.x86-64` property in a cinterop .def file is not actually used
by cinterop. `headers.x64` should be used instead.

This commit removes it completely instead of renaming -- no functional
changes are done.

kotlin-native/platformLibs/src/platform/linux/linux.def

@@ -33,8 +33,6 @@ headers = aio.h aliases.h a.out.h argp.h argz.h byteswap.h cpio.h crypt.h \
 	sys/ustat.h utime.h sys/utsname.h sys/vfs.h sys/vlimit.h \
 	sys/vt.h sys/vtimes.h sys/xattr.h
 
-headers.x86-64 = sys/debugreg.h sys/reg.h sys/io.h sys/perm.h sys/kdaemon.h
-
 compilerOpts = -D_ANSI_SOURCE -D_POSIX_C_SOURCE=199309 -D_BSD_SOURCE -D_XOPEN_SOURCE=700
 depends = posix