syscall: remove comment about test vs bt x86 instructions

According to Agner Fog's documentation
(https://agner.org/optimize/instruction_tables.pdf), the following information
is true:

These instructions have identical latency and throughput, except for Zen
5, which lists `test` as having reciprocal throughput of 0.25 cycles,
and `bt` having reciprocal throughput of 0.33 cycles. This rules out the
`bt` instruction for being the ideal instruction.

The other `test` instructions were removed as candidates because,
regardless of the size of the memory fetched at the address, at least 64
bytes will need to be fetched because it will be stored in the cache
line. The WORD and DWORD cases can be ruled out because we cannot assume
that `rsp + 16` or `rsp + 17` will not be on a 64-byte alignment
boundary, which would cause two cachelines to be essentially filled with
garbage we don't care about. The best case scenario is that we only need
to fill one cache line with garbage, which is what the BYTE version does
every time.

src/arch/x86_64/interrupt/syscall.rs

@@ -144,18 +144,6 @@ pub unsafe extern "C" fn syscall_instruction() {
     // instruction, whereas debuggers expect the iretq behavior of returning to after the
     // instruction.
 
-    // TODO: Which one is faster?
-    //      bt DWORD PTR [rsp + 16], 8
-    //  or,
-    //      bt BYTE PTR [rsp + 17], 0
-    //  or,
-    //      test BYTE PTR [rsp + 17], 1
-    //  or,
-    //      test WORD PTR [rsp + 16], 0x100
-    //  or,
-    //      test DWORD PTR [rsp + 16], 0x100
-    //  ?
-
     "test BYTE PTR [rsp + 17], 1;",
     // If set, return using IRETQ instead.
     "jnz 2f;",