How can I remove a specific part of an image?

You can find the index of the black circle and assign a value to the pixel to the left or right of the black circle. Here is the sample code for this

import cv2
import numpy as np

# read the image
img = cv2.imread('original.png')
cv2.imshow("Image", img)

# convert image to numpy array and also to grayscale
img = np.array(img)
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

# get height and width of image
[rows, cols] = gray.shape

# now extract one row from image, find indices of black circle
# and make those pixels white which are to the left/right
# of black cirlce
for i in range(rows):
    row = gray[i, :] # extract row of image
    indices = np.where(row == 0)    # find indices of black circle
    indices = indices[0]

    # if indices are not empty
    if len(indices) > 0:
        # find starting/ending column index
        si = indices[0]
        ei = indices[len(indices)-1]

        # assign values to the range of pixels
        img[i, 0:si-1] = [255, 255, 255]
        img[i, ei+1:] = [255, 255, 255]
    # if indices is empty then make whole row white
    else:
        img[i,:] = [255, 255, 255]

cv2.imshow("Modified Image", img)
cv2.waitKey(0)
cv2.destroyAllWindows()

input image

output image