[AGIPD][CORRECT] disable rel_gain with fixed gain and remove unused parameter

xfel-calibrate AGIPD CORRECT \
--in-folder /gpfs/exfel/d/raw/SPB/202130/p900188 \
--out-folder /gpfs/exfel/data/scratch/ahmedk/test/AGIPD_SPB2 \
--run 263 --sequences 0 \
--karabo-da-control AGIPD1MCTRL00 \
--karabo-id-control SPB_IRU_AGIPD1M1 \
--h5path-ctrl /CONTROL/{}/MDL/FPGA_COMP \
--karabo-id SPB_DET_AGIPD1M-1 \
--slurm-mem 750

# Correction Booleans
only_offset = False # Apply only Offset correction. if False, Offset is applied by Default. if True, Offset is only applied.
rel_gain = True # do relative gain correction based on PC data
xray_gain = True # do relative gain correction based on xray data
blc_noise = True # if set, baseline correction via noise peak location is attempted
blc_stripes = True # if set, baseline corrected via stripes
blc_hmatch = True # if set, base line correction via histogram matching is attempted
match_asics = True # if set, inner ASIC borders are matched to the same signal level
adjust_mg_baseline = True # adjust medium gain baseline to match highest high gain value
zero_nans = True # set NaN values in corrected data to 0
zero_orange = True # set to 0 very negative and very large values in corrected data
blc_set_min = True # Shift to 0 negative medium gain pixels after offset corr
corr_asic_diag = False # if set, diagonal drop offs on ASICs are correted
force_hg_if_below = True # set high gain if mg offset subtracted value is below hg_hard_threshold
force_mg_if_below = True # set medium gain if mg offset subtracted value is below mg_hard_threshold
mask_noisy_adc = True # Mask entire ADC if they are noise above a relative threshold
common_mode = True # Common mode correction
melt_snow = True # Identify (and optionally interpolate) 'snowy' pixels
mask_zero_std = True # Mask pixels with zero standard deviation across train
low_medium_gap = True # 5 sigma separation in thresholding between low and medium gain