Remove incorrect ARMBUILD_ONLY from device code *.proj

[cw2]

The following device code *.proj files in \DeviceCode\Targets\Native

STM32\DeviceCode\STM32_Bootstrap\dotNetMF.proj
STM32\DeviceCode\STM32_I2C\dotNetMF.proj
STM32\DeviceCode\STM32_IntC\dotNetMF.proj
STM32F4\DeviceCode\STM32F4_Bootstrap\dotNetMF.proj
STM32F4\DeviceCode\STM32F4_I2C\dotNetMF.proj
STM32F4\DeviceCode\STM32F4_IntC\dotNetMF.proj
STM32F4\DeviceCode\STM32F4_security\dotNetMF.proj

contain

<PropertyGroup>
  <ARMBUILD_ONLY>true</ARMBUILD_ONLY>
</PropertyGroup>

which is used in the GCC targets file to conditionally specify -mthumb option

<CC_CPP_COMMON_FLAGS Condition="'$(INSTRUCTION_SET)'=='thumb' and '$(ARMBUILD_ONLY)'!='true' ">... -mthumb</...>    

However, the Cortex-M series do not support ARM instruction set, so setting in the proj file is incorrect and should be removed. The -mthumb option is specified in the ARCH_TYPE_FLAGS tag based on the device type, making the above statement redundant.