Drop u-pre's font-weight

When displaying file contents, as for example on [*], u-pre, setting
font weight to 500, is combined with u-monospace, setting font-family
to `Source Code Pro`.

The @import of `Source Code Pro` only defines the font face for font
weight 400. So on setups without that font, the 500 has no effect and
display falls back to using `Source Code Pro` with the lighter weight
400 (i.e., `normal`).

On setups where the monospace font is provided with both 400 and 500
weights (users with the fonts installed locally, or using Gerrit's own
provided fonts setup), the heavier 500 weight gets used, leading to an
undesirable bold look for source code.

Drop u-pre's font-weight: this way, we do not change the appearance on
most setups and we avoid bold-looking source code on setups that
provide their own fonts.

[*] https://gerrit.googlesource.com/gerrit/+/589af813ccca4c329bd4845852cd9d6d73d1d11d/gerrit-main/src/main/java/Main.java

Change-Id: I7322c2762f6bce1a6fc6b0de1a2a3674092ef8a1

resources/com/google/gitiles/static/base.css

diff --git a/resources/com/google/gitiles/static/base.css b/resources/com/google/gitiles/static/base.css
index 37a98c4..9719920 100644
--- a/resources/com/google/gitiles/static/base.css
+++ b/resources/com/google/gitiles/static/base.css
@@ -55,7 +55,6 @@
 }
 .u-pre {
   font-size: 10pt;
-  font-weight: 500;
   white-space: pre;
 }
 .u-lineNum {