build: Drop CONFIG_SPL_BUILD guards in some cases

Given gcc-6.1 and later we can now safely have strings discarded when
the functions are unused.  This lets us drop certain cases of not
building something so that we don't have the strings brought in when the
code was discarded.  Simplify the code now by dropping guards we don't
need now.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Chander Kashyap <k.chander@samsung.com>
Cc: Thomas Abraham <thomas.ab@samsung.com>
Cc: Vipin Kumar <vipin.kumar@st.com>
Cc: Wenyou Yang <wenyou.yang@microchip.com>
Signed-off-by: Tom Rini <trini@konsulko.com>