stm32f1: remove stm32f1 support

A few years ago STM32F1 SoCs support has been added :
0144caf22ce6acd5c  gpio: stm32: add stm32f1 support
2d18ef2364fd3561a  ARMv7M: add STM32F1 support

But neither STM32F1 dedicated defconfig nor board was
associated to these commits.

Got confirmation from Tom Rini and Matt Porter to remove
all this code [1]

[1] http://u-boot.10912.n7.nabble.com/Remove-STM32F1-support-td301603.html

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Tom Rini <trini@konsulko.com>