Auto merge of #29546 - mrobinson:fix-build-warnings, r=jdm

Fix some build warnings

Change a use of `extern crate` to be simply `use`, which is what the compiler suggests. In addition, with the change, one of the uses is redundant. In addition, remove two unused `use` statements.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they fix some warnings.

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components/hashglobe/src/alloc.rs

@@ -27,7 +27,7 @@ pub use self::platform::{alloc, dealloc, realloc};
 
 #[cfg(any(unix, target_os = "redox"))]
 mod platform {
-    extern crate libc;
+    use libc;
 
     #[cfg(not(any(target_os = "android")))]
     use std::ptr;

components/hashglobe/src/table.rs

@@ -724,7 +724,6 @@ fn test_offset_calculation() {
 
 impl<K, V> RawTable<K, V> {
     unsafe fn new_uninitialized(capacity: usize) -> RawTable<K, V> {
-        extern crate libc;
         if let Ok(table) = Self::try_new_uninitialized(capacity) {
             table
         } else {

components/layout_thread/lib.rs

@@ -63,7 +63,6 @@ use layout::traversal::{
 };
 use layout::wrapper::LayoutNodeLayoutData;
 use layout_traits::LayoutThreadFactory;
-use libc::c_void;
 use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use metrics::{PaintTimeMetrics, ProfilerMetadataFactory, ProgressiveWebMetric};
 use msg::constellation_msg::{

components/layout_thread_2020/lib.rs

@@ -48,7 +48,6 @@ use layout::query::{
 use layout::traversal::RecalcStyle;
 use layout::{BoxTree, FragmentTree};
 use layout_traits::LayoutThreadFactory;
-use libc::c_void;
 use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use metrics::{PaintTimeMetrics, ProfilerMetadataFactory, ProgressiveWebMetric};
 use msg::constellation_msg::{