Remove C90 requirement from CMakeLists (#649)

This is needed as it is breaking projects - https://forums.freertos.org/t/freertos-gcc-cmake/16984

We will re-evaluate and accordingly add this later.

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6d83db8..d45de64 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -292,5 +292,3 @@
 )
 
 ########################################################################
-# Requirements
-set_property(TARGET freertos_kernel PROPERTY C_STANDARD 90)