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

docs/ChangeLog/20190830/PR4871.md

@@ -1,4 +0,0 @@
-* LUFA USB descriptor cleanup
-    * Some code cleanups related to the USB HID descriptors on AVR keyboards, to make them easier to read and understand
-    * More information: see https://github.com/qmk/qmk_firmware/pull/4871
-    * No behaviour changes anticipated and no keymaps modified

docs/ChangeLog/20190830/PR6245.md

@@ -1,5 +0,0 @@
-* Update repo to use LUFA as a git submodule
-  * `/lib/LUFA` removed from the repo
-  * LUFA set as a submodule, pointing to qmk/lufa
-  * This should allow more flexibility with LUFA, and allow us to keep the sub-module up to date, a lot more easily.  It was ~2 years out of date with no easy path to fix that.  This prevents that from being an issue in the future
-  

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 */