soc: qcom: icc-bwmon: Remove unused struct member

bwmon->regmap was never used, as the regmap for bwmon is registered
through devres and accessed through bwmon's regmap_field members.
Remove it

Fixes: ec63dcd3 ("soc: qcom: icc-bwmon: use regmap and prepare for BWMON v5")
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230304-topic-ddr_bwmon-v3-2-77a050c2fbda@linaro.org