clean(old): remove unused file

src/postprocessor/core/export_pdf.py

-#!/usr/bin/env python3
-import numpy as np
-from matplotlib import pyplot as plt
-from matplotlib.backends.backend_pdf import PdfPages
-
-
-def dummyplot():
-    plt.plot(np.random.randint(10, size=5), np.random.randint(10, size=5))
-
-
-plt1 = dummyplot()
-plt2 = dummyplot()
-pp = PdfPages("foo.pdf")
-for i in [plt1, plt2]:
-    pp.savefig(i)
-pp.close()