SIMD: Formatting tweaks + remove unnecessary code

+ "JSIMD_ARM_NEON" = "JSIMD_NEON"
+ "JSIMD_MIPS_DSPR2" = "JSIMD_DSPR2"
+ "*_mips_dspr2" = "*_dspr2"

It's obvious that "NEON" refers to Arm and "DSPr2" refers to MIPS, and
this naming convention is consistent with the other SIMD extensions.

simd/mips/jsimd_dspr2_asm.h

@@ -112,9 +112,9 @@ symbol:         .frame  sp, 0, ra;                      \
                 .set    noat;
 
 /*
- * LEAF_MIPS_DSPR2 - declare leaf routine for MIPS DSPr2
+ * LEAF_DSPR2 - declare leaf routine for MIPS DSPr2
  */
-#define LEAF_MIPS_DSPR2(symbol)                         \
+#define LEAF_DSPR2(symbol)                              \
 LEAF_MIPS32R2(symbol)                                   \
                 .set    dspr2;