Removed prescaler define from avr i2c, as it was impossible to use (#6617)

docs/i2c_driver.md

@@ -34,7 +34,6 @@ The following defines can be used to configure the I2C master driver.
 |Variable          |Description                                        |Default|
 |------------------|---------------------------------------------------|-------|
 |`F_SCL`           |Clock frequency in Hz                              |400KHz |
-|`Prescaler`       |Divides master clock to aid in I2C clock selection |1      |
 
 AVRs usually have set GPIO which turn into I2C pins, therefore no further configuration is required.
 

drivers/avr/i2c_master.c

@@ -27,8 +27,8 @@
 #ifndef F_SCL
 #    define F_SCL 400000UL  // SCL frequency
 #endif
-#define Prescaler 1
-#define TWBR_val ((((F_CPU / F_SCL) / Prescaler) - 16) / 2)
+
+#define TWBR_val (((F_CPU / F_SCL) - 16) / 2)
 
 void i2c_init(void) {
     TWSR = 0; /* no prescaler */