iar - remove --vla flag for exporters

[0xc0170]

The template file already enables VLA as it's for C only. This --vla flag
causes conflicts with --cpp flag that is enabled implicitly if C++ is enabled.

IAR template before this patch:

          <state></state>
          <state>--guard_calls</state>
          <state>--preinclude=mbed_config.h</state>
          <state>--vla</state>
          <state>--preinclude=mbed_config.h</state>
          <name>IExtraOptions</name>

This causes problems because C++ is enabled, thus --vla causes errors when compiling.

IAR template after this patch:

          <state></state>
          <state>--guard_calls</state>
          <state>--preinclude=mbed_config.h</state>
          <state>--preinclude=mbed_config.h</state>
          <name>IExtraOptions</name>

@theotherjimmy Seems that preinclude is 2x there

@sg- @bogdanm

[screamerbg]

@0xc0170 does this mean that --vla is enabled by default in IAR Workbench. And the more general question, if I compile code with IAR compiler (mbed build system) and with IAR workbench (mbed exporters), will I get same result/output?