Remove incorrect i386 regs: dil/sil/spl/bpl

Those regs does not seem to work on i386: I can't do `i r dil` in i386 but I can do so in amd64 binaries.

Via https://www.tortall.net/projects/yasm/manual/html/arch-x86-registers.html :
```
The 64-bit x86 register set consists of 16 general purpose registers, only 8 of which are available in 16-bit and 32-bit mode. The core eight 16-bit registers are AX, BX, CX, DX, SI, DI, BP, and SP. The least significant 8 bits of the first four of these registers are accessible via the AL, BL, CL, and DL in all execution modes. In 64-bit mode, the least significant 8 bits of the other four of these registers are also accessible; these are named SIL, DIL, SPL, and BPL. The most significant 8 bits of the first four 16-bit registers are also available, although there are some restrictions on when they can be used in 64-bit mode; these are named AH, BH, CH, and DH.
```

and the table present there, it seems SIL, DIL, SPL and BPL are only available in 64-bit mode.

pwndbg/regs.py

@@ -157,7 +157,6 @@ i386 = RegisterSet( pc      = 'eip',
                                 'bx','bh','bl',
                                 'cx','ch','cl',
                                 'dx','dh','dl',
-                                'dil','sil','spl','bpl',
                                 'di','si','bp','sp','ip'),
                     retval  = 'eax')