docs: do not document CT_KERNEL_ARCH, it's going away...

Since we merged the 32/64-bit archs, and we explicitly used
the Linux kernel arch name, we can get rid of CT_KERNEL_ARCH.

docs/overview.txt

@@ -826,17 +826,6 @@ The architecture's ".sh" file API:
      - defaults to:
        - for glibc-based toolchain: "gnu"
        - for uClibc-based toolchain: "uclibc"
-   + provides:
-     - optional
-     - the environment variable CT_KERNEL_ARCH
-     - contains:
-       the architecture name as understandable by the Linux kernel build
-       system.
-       Eg.: "arm" for an ARM
-            "powerpc" for a PowerPC
-            "i386" for an x86
-     - defaults to:
-       ${CT_ARCH}
    + provides:
      - optional
      - the environment variables to configure the cross-gcc (defaults)