famec: eliminate global context ptr

saves like 25-35K of .text

current compile resource usage on i5-6600K:
            cpu  mem
gcc 5.4.0:  17.0 1.1g
clang 3.8:  1686 2.3g

FAMEC_NO_GOTOS:
gcc 5.4.0:   8.4 0.4g
clang 3.8:  20.0 0.15g
vs2008/O2: ~1800 ?
vs2008/O1:  ~720 ?

pico/cd/mcd.c

@@ -139,7 +139,6 @@ static void SekRunS68k(unsigned int to)
   m68k_set_context(&PicoCpuMM68k);
 #elif defined(EMU_F68K)
   SekCycleCntS68k += fm68k_emulate(&PicoCpuFS68k, cyc_do, 0) - cyc_do;
-  g_m68kcontext = &PicoCpuFM68k;
 #endif
 }