Remove comment about a branch being optimized out, fix rust-lang#45831

Most often, this code is used through the `std::heap::Heap`
and `#[gloabal_allocator]` indirection, so this branch is not
optimized out anymore.

Author: SimonSapin

src/liballoc_jemalloc/lib.rs

@@ -72,8 +72,7 @@ mod contents {
     const MALLOCX_ZERO: c_int = 0x40;
 
     // The minimum alignment guaranteed by the architecture. This value is used to
-    // add fast paths for low alignment values. In practice, the alignment is a
-    // constant at the call site and the branch will be optimized out.
+    // add fast paths for low alignment values.
     #[cfg(all(any(target_arch = "arm",
                   target_arch = "mips",
                   target_arch = "powerpc")))]

src/liballoc_system/lib.rs

@@ -25,8 +25,7 @@
 #![rustc_alloc_kind = "lib"]
 
 // The minimum alignment guaranteed by the architecture. This value is used to
-// add fast paths for low alignment values. In practice, the alignment is a
-// constant at the call site and the branch will be optimized out.
+// add fast paths for low alignment values.
 #[cfg(all(any(target_arch = "x86",
               target_arch = "arm",
               target_arch = "mips",