arm-bsp/hafnium: update bbappend for 2.6

No need for this to be versioned as it complicates upgrades.

Remove the explicit post-2.5 SRCREV now that the recipe has upgraded to
2.6, and remove assignments which are already the default for
conciseness.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm-bsp/recipes-bsp/hafnium/hafnium-tc.inc

@@ -1,8 +1,4 @@
 # TC0 specific configuration
 
 COMPATIBLE_MACHINE = "(tc?)"
-HAFNIUM_PROJECT = "reference"
-HAFNIUM_INSTALL_TARGET = "hafnium"
 HAFNIUM_PLATFORM = "secure_tc"
-
-SRCREV = "464fa5a287791e7d128e37721a7d1da257144e12"