Toolchain: Remove check for default_lib attribute

Unfortunately the original fix doesn't hold for all targets in the online
compiler. I think this is because when we override target.c_lib with
target.default_lib, the default_lib isn't in the target.supported_c_libs
list. Removing the default_lib patch altogether seems to work in the
sense that compilation works. This may cause issues down the line if the
tools inadevertently swap out the user's selected c_lib with something
else. This is another problem we have to solve in a generic way which
supports all use cases we want for offline and online tools. At the
moment this is just a patch to get things working for mbed 6.

Author: rwalton-arm

tools/toolchains/mbed_toolchain.py

@@ -1097,11 +1097,6 @@ def check_c_lib_supported(self, target, toolchain):
         target.c_lib is modified to have the lowercased string of its original string.
         This is done to be case insensitive when validating.
         """
-        if hasattr(target, "default_lib"):
-            # Use default_lib as the c_lib attribute. This allows backwards
-            # compatibility with older target definitions, allowing either
-            # default_lib or c_lib to specify the C library to use.
-            target.c_lib = target.default_lib.lower()
         if hasattr(target, "c_lib"):
             target.c_lib = target.c_lib.lower()
             if (