tcg/ppc: Remove unused constraints A, B, C, D

These constraints have not been used for quite some time.

Fixes: 77b73de ("Use rem/div[u]_i32 drop div[u]2_i32")
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tcg/ppc/tcg-target-con-str.h

@@ -10,10 +10,6 @@
  */
 REGS('r', ALL_GENERAL_REGS)
 REGS('v', ALL_VECTOR_REGS)
-REGS('A', 1u << TCG_REG_R3)
-REGS('B', 1u << TCG_REG_R4)
-REGS('C', 1u << TCG_REG_R5)
-REGS('D', 1u << TCG_REG_R6)
 
 /*
  * Define constraint letters for constants: