Revert getCpuFreqMHz(), because of missing IDF support.

Author: everslick

cores/esp32/Esp.h

@@ -58,7 +58,7 @@ class EspClass
     void restart();
     uint32_t getFreeHeap();
     uint8_t getChipRevision();
-    uint32_t getCpuFreqMHz() { return getCpuFrequencyMhz(); }
+    uint32_t getCpuFreqMHz() { return CONFIG_ESP32_DEFAULT_CPU_FREQ_MHZ; }
     uint32_t getCycleCount();
     const char * getSdkVersion();