Fix duplicate maybe_uninit_extra attribute

Author: Jethro Beekman

src/libstd/lib.rs

@@ -224,10 +224,7 @@
     all(target_vendor = "fortanix", target_env = "sgx"),
     feature(slice_index_methods, coerce_unsized, sgx_platform, ptr_wrapping_offset_from)
 )]
-#![cfg_attr(
-    all(test, target_vendor = "fortanix", target_env = "sgx"),
-    feature(fixed_size_array, maybe_uninit_extra)
-)]
+#![cfg_attr(all(test, target_vendor = "fortanix", target_env = "sgx"), feature(fixed_size_array))]
 // std is implemented with unstable features, many of which are internal
 // compiler details that will never be stable
 // NB: the following list is sorted to minimize merge conflicts.