Remove toolsDependencies entries

This change is required due to a bug in the Arduino IDE which causes the
tool versions specified by avr_boot to be used by other platforms such
as Arduino AVR Boards after Boards Manager installation of avr_boot(see
https://github.com/arduino/Arduino/issues/5168). A specific issue caused
by this is that Arduino AVR Boards 1.6.12  requires LTO support, which
avr-gcc does not have. This will cause Arduino AVR Boards to no longer
compile when the previous version of package_zevero_avr_boot_index.json
was used to install avr_boot.

Removing the toolsDependencies entries causes avr_boot to use whatever
tool versions were previously installed.

The disadvantages of this solution are:

- Breaks compatibility with the Eclipse Arduino plugin(see
Sloeber/arduino-eclipse-plugin#516), which
requires these entries.

- Loss of control over the specific toolchain used.

Author: per1234

package_zevero_avr_boot_index.json

@@ -26,18 +26,7 @@
             {"name": "ATmega32U4"},
             {"name": "ATmega1284P"}
           ],
-          "toolsDependencies": [
-            {
-              "packager": "arduino",
-              "name": "avr-gcc",
-              "version": "4.8.1-arduino5"
-            },
-            {
-              "packager": "arduino",
-              "name": "avrdude",
-              "version": "6.0.1-arduino5"
-            }
-          ]
+          "toolsDependencies": []
         }
       ],
       "tools": []